5 Hypervisor Extension, Version 1.0
This chapter describes the RISC-V hypervisor extension, which virtualizes the supervisor-level architecture to support the efficient hosting of guest operating systems atop a type-1 or type-2 hypervisor. The hypervisor extension changes supervisor mode into hypervisor-extended supervisor mode (HS-mode, or hypervisor mode for short), where a hypervisor or a hosting-capable operating system runs. The hypervisor extension also adds another stage of address translation, from guest physical addresses to supervisor physical addresses, to virtualize the memory and memory-mapped I/O subsystems for a guest operating system. HS-mode acts the same as S-mode, but with additional instructions and CSRs that control the new stage of address translation and support hosting a guest OS in virtual S-mode (VS-mode). Regular S-mode operating systems can execute without modification either in HS-mode or as VS-mode guests.
In HS-mode, an OS or hypervisor interacts with the machine through the same SBI as an OS normally does from S-mode. An HS-mode hypervisor is expected to implement the SBI for its VS-mode guest.
The hypervisor extension depends on an “I” base integer ISA with
32 x registers (RV32I or RV64I), not RV32E, which has only
16 x registers.
CSR mtval must not be read-only zero, and
standard page-based address translation must be supported, either
Sv32 for RV32, or a minimum of Sv39 for RV64.
The hypervisor extension is enabled by setting bit 7 in the misa CSR,
which corresponds to the letter H.
RISC-V harts that implement the hypervisor extension are encouraged
not to hardwire misa[7], so that the extension may be disabled.
5.1 Privilege Modes
The current virtualization mode, denoted V, indicates whether the hart is currently executing in a guest. When V=1, the hart is either in virtual S-mode (VS-mode), or in virtual U-mode (VU-mode) atop a guest OS running in VS-mode. When V=0, the hart is either in M-mode, in HS-mode, or in U-mode atop an OS running in HS-mode. The virtualization mode also indicates whether two-stage address translation is active (V=1) or inactive (V=0). Table 1.1 lists the possible privilege modes of a RISC-V hart with the hypervisor extension.
For privilege modes U and VU, the nominal privilege mode is U, and for privilege modes HS and VS, the nominal privilege mode is S.
HS-mode is more privileged than VS-mode, and VS-mode is more privileged than VU-mode. VS-mode interrupts are globally disabled when executing in U-mode.
This description does not consider the possibility of U-mode or VU-mode interrupts and will be revised if an extension for user-level interrupts is adopted.
5.2 Hypervisor and Virtual Supervisor CSRs
An OS or hypervisor running in HS-mode uses the supervisor CSRs to interact with the exception,
interrupt, and address-translation subsystems.
Additional CSRs are provided to HS-mode, but not to VS-mode, to manage
two-stage address translation and to control the behavior of a VS-mode guest:
hstatus, hedeleg, hideleg, hvip, hip, hie,
hgeip, hgeie, henvcfg, henvcfgh,
hcounteren, htimedelta, htimedeltah, htval,
htinst, and hgatp.
Furthermore, several virtual supervisor CSRs (VS CSRs) are replicas
of the normal supervisor CSRs.
For example, vsstatus is the VS CSR that duplicates the usual
sstatus CSR.
When V=1, the VS CSRs substitute for the corresponding supervisor CSRs, taking over all functions of the usual supervisor CSRs except as specified otherwise. Instructions that normally read or modify a supervisor CSR shall instead access the corresponding VS CSR. When V=1, an attempt to read or write a VS CSR directly by its own separate CSR address causes a virtual instruction exception. (Attempts from U-mode cause an illegal instruction exception as usual.) The VS CSRs can be accessed as themselves only from M-mode or HS-mode.
While V=1, the normal HS-level supervisor CSRs that are replaced by VS CSRs retain their values but do not affect the behavior of the machine unless specifically documented to do so. Conversely, when V=0, the VS CSRs do not ordinarily affect the behavior of the machine other than being readable and writable by CSR instructions.
Some standard supervisor CSRs (senvcfg,
scounteren, and scontext,
possibly others) have no matching VS CSR.
These supervisor CSRs continue to have their usual function and
accessibility even when V=1, except with VS-mode and VU-mode substituting for
HS-mode and U-mode.
Hypervisor software is expected to manually swap the contents of these
registers as needed.
Matching VS CSRs exist only for the supervisor CSRs that must be duplicated, which are mainly those that get automatically written by traps or that impact instruction execution immediately after trap entry and/or right before SRET, when software alone is unable to swap a CSR at exactly the right moment. Currently, most supervisor CSRs fall into this category, but future ones might not.
In this chapter, we use the term HSXLEN to refer to the effective XLEN when executing in HS-mode, and VSXLEN to refer to the effective XLEN when executing in VS-mode.
5.2.1 Hypervisor Status Register (hstatus)
The hstatus register is an HSXLEN-bit read/write register
formatted as shown in Figure 1.2 when HSXLEN=32 and
Figure 1.3 when HSXLEN=64.
The hstatus
register provides facilities analogous to the mstatus register
for tracking and controlling the exception behavior of a VS-mode guest.
hstatus) when HSXLEN=32.
hstatus) when HSXLEN=64.The VSXL field controls the effective XLEN for VS-mode (known as VSXLEN),
which may differ from the XLEN for HS-mode (HSXLEN).
When HSXLEN=32, the VSXL field does not exist, and VSXLEN=32.
When HSXLEN=64, VSXL is a WARL field that is encoded the same as the
MXL field of misa, shown in Table [misabase] on
page .
In particular, an implementation may make VSXL be a read-only field whose
value always ensures that VSXLEN=HSXLEN.
If HSXLEN is changed from 32 to a wider width, and if field VSXL is not restricted to a single value, it gets the value corresponding to the widest supported width not wider than the new HSXLEN.
The hstatus fields VTSR, VTW, and VTVM are defined analogously to the
mstatus fields TSR, TW, and TVM, but affect execution only in VS-mode,
and cause virtual instruction exceptions instead of illegal instruction
exceptions.
When VTSR=1, an attempt in VS-mode to execute SRET raises a virtual
instruction exception.
When VTW=1 (and assuming mstatus.TW=0), an attempt in VS-mode to
execute WFI raises a virtual instruction exception if the WFI does not
complete within an implementation-specific, bounded time limit.
When VTVM=1, an attempt in VS-mode to execute SFENCE.VMA or SINVAL.VMA or to
access CSR satp raises a virtual instruction exception.
The VGEIN (Virtual Guest External Interrupt Number) field selects a guest external interrupt source for VS-level external interrupts. VGEIN is a WLRL field that must be able to hold values between zero and the maximum guest external interrupt number (known as GEILEN), inclusive. When VGEIN=0, no guest external interrupt source is selected for VS-level external interrupts. GEILEN may be zero, in which case VGEIN may be read-only zero. Guest external interrupts are explained in Section 1.2.4, and the use of VGEIN is covered further in Section 1.2.3.
Field HU (Hypervisor in U-mode) controls whether the virtual-machine load/store instructions, HLV, HLVX, and HSV, can be used also in U-mode. When HU=1, these instructions can be executed in U-mode the same as in HS-mode. When HU=0, all hypervisor instructions cause an illegal instruction trap in U-mode.
The HU bit allows a portion of a hypervisor to be run in U-mode for greater protection against software bugs, while still retaining access to a virtual machine’s memory.
The SPV bit (Supervisor Previous Virtualization mode) is written by the implementation
whenever a trap is taken into HS-mode.
Just as the SPP bit in sstatus is set to the (nominal) privilege
mode at the time of the trap, the SPV bit in hstatus is set to the value of the virtualization
mode V at the time of the trap. When an SRET instruction is executed when V=0,
V is set to SPV.
When V=1 and a trap is taken into HS-mode, bit SPVP (Supervisor Previous
Virtual Privilege) is set to the nominal privilege mode at the time of the trap,
the same as sstatus.SPP.
But if V=0 before a trap, SPVP is left unchanged on trap entry.
SPVP controls the effective privilege of explicit memory accesses made by
the virtual-machine load/store instructions, HLV, HLVX, and HSV.
Without SPVP, if instructions HLV, HLVX, and HSV looked instead to
sstatus.SPP for the effective privilege of their memory accesses,
then, even with HU=1, U-mode could not access virtual machine memory at
VS-level, because to enter U-mode using SRET always leaves SPP=0.
Unlike SPP, field SPVP is untouched by transitions back-and-forth between
HS-mode and U-mode.
Field GVA (Guest Virtual Address) is written by the implementation
whenever a trap is taken into HS-mode.
For any trap (breakpoint, address misaligned,
access fault, page fault, or guest-page fault) that writes
a guest virtual address to stval, GVA is set to 1.
For any other trap into HS-mode, GVA is set to 0.
For breakpoint and memory access traps
that write a nonzero value to stval,
GVA is redundant with field SPV (the two bits are set
the same) except when the explicit memory access of an HLV, HLVX, or HSV
instruction causes a fault.
In that case, SPV=0 but GVA=1.
The VSBE bit is a WARL field that controls the endianness of explicit memory accesses made from VS-mode. If VSBE=0, explicit load and store memory accesses made from VS-mode are little-endian, and if VSBE=1, they are big-endian. VSBE also controls the endianness of all implicit accesses to VS-level memory management data structures, such as page tables. An implementation may make VSBE a read-only field that always specifies the same endianness as HS-mode.
5.2.2 Hypervisor Trap Delegation Registers (hedeleg and hideleg)
Registers hedeleg and hideleg are HSXLEN-bit read/write
registers, formatted as shown in Figures 1.4 and
1.5 respectively.
By default, all traps at any privilege level are handled in M-mode, though
M-mode usually uses the medeleg and mideleg CSRs to delegate
some traps to HS-mode. The hedeleg and hideleg CSRs allow these
traps to be further delegated to a VS-mode guest; their layout is the same
as medeleg and mideleg.
hedeleg).
hideleg).| Bit | Attribute | Corresponding Exception |
|---|---|---|
| 0 | (See text) | Instruction address misaligned |
| 1 | Writable | Instruction access fault |
| 2 | Writable | Illegal instruction |
| 3 | Writable | Breakpoint |
| 4 | Writable | Load address misaligned |
| 5 | Writable | Load access fault |
| 6 | Writable | Store/AMO address misaligned |
| 7 | Writable | Store/AMO access fault |
| 8 | Writable | Environment call from U-mode or VU-mode |
| 9 | Read-only 0 | Environment call from HS-mode |
| 10 | Read-only 0 | Environment call from VS-mode |
| 11 | Read-only 0 | Environment call from M-mode |
| 12 | Writable | Instruction page fault |
| 13 | Writable | Load page fault |
| 15 | Writable | Store/AMO page fault |
| 20 | Read-only 0 | Instruction guest-page fault |
| 21 | Read-only 0 | Load guest-page fault |
| 22 | Read-only 0 | Virtual instruction |
| 23 | Read-only 0 | Store/AMO guest-page fault |
A synchronous trap that has been delegated to HS-mode (using
medeleg) is further delegated to VS-mode if V=1 before the trap and
the corresponding hedeleg bit is set.
Each bit of hedeleg shall be either writable or read-only zero.
Many bits of hedeleg are required specifically to be writable or
zero, as enumerated in Table [tab:hedeleg-bits].
Bit 0, corresponding to instruction address misaligned exceptions, must
be writable if IALIGN=32.
Requiring that certain bits of hedeleg be writable reduces some of
the burden on a hypervisor to handle variations of implementation.
An interrupt that has been delegated to HS-mode (using mideleg) is
further delegated to VS-mode if the corresponding hideleg bit is
set.
Among bits 15:0 of hideleg, bits 10, 6, and 2 (corresponding
to the standard VS-level interrupts) are writable, and bits 12, 9, 5,
and 1 (corresponding to the standard S-level interrupts) are read-only
zeros.
When a virtual supervisor external interrupt (code 10) is delegated to
VS-mode, it is automatically translated by the machine into a supervisor
external interrupt (code 9) for VS-mode, including the value written to
vscause on an interrupt trap.
Likewise, a virtual supervisor timer interrupt (6) is translated into a
supervisor timer interrupt (5) for VS-mode, and a virtual supervisor
software interrupt (2) is translated into a supervisor software interrupt
(1) for VS-mode.
Similar translations may or may not be done for platform or custom
interrupt causes (codes 16 and above).
5.2.3 Hypervisor Interrupt Registers (hvip, hip, and hie)
Register hvip is an HSXLEN-bit read/write register that a
hypervisor can write to indicate virtual interrupts intended for VS-mode.
Bits of hvip that are not writable are read-only zeros.
hvip).The standard portion (bits 15:0) of hvip is formatted as shown in
Figure 1.7.
Bits VSEIP, VSTIP, and VSSIP of hvip are writable.
Setting VSEIP=1 in hvip asserts a VS-level external interrupt;
setting VSTIP asserts a VS-level timer interrupt; and setting VSSIP
asserts a VS-level software interrupt.
hvip.Registers hip and hie are HSXLEN-bit read/write registers
that supplement HS-level’s sip and sie respectively.
The hip register indicates pending VS-level and hypervisor-specific
interrupts, while hie contains enable bits for the same interrupts.
hip).
hie).For each writable bit in sie, the corresponding bit shall be
read-only zero in both hip and hie.
Hence, the nonzero bits in sie and hie are always mutually
exclusive, and likewise for sip and hip.
The active bits of hip and hie cannot be placed in HS-level’s
sip and sie because doing so would make it impossible for
software to emulate the hypervisor extension on platforms that do not
implement it in hardware.
An interrupt i will trap to HS-mode whenever all of the
following are true:
(a) either the current operating mode is HS-mode and the SIE bit in the
sstatus register is set, or the current operating mode has less
privilege than HS-mode;
(b) bit i is set in both sip and sie, or in both
hip and hie; and
(c) bit i is not set in hideleg.
If bit i of sie is read-only zero, the same bit in
register hip may be writable or may be read-only.
When bit i in hip is writable, a pending interrupt
i can be cleared by writing 0 to this bit.
If interrupt i can become pending in hip but
bit i in hip is read-only, then either
the interrupt can be cleared by clearing bit i
of hvip, or the implementation must provide
some other mechanism for clearing the pending interrupt (which may
involve a call to the execution environment).
A bit in hie shall be writable if the corresponding interrupt can
ever become pending in hip.
Bits of hie that are not writable shall be read-only zero.
The standard portions (bits 15:0) of registers hip and hie
are formatted as shown in Figures 1.10 and
1.11 respectively.
hip.
hie.Bits hip.SGEIP and hie.SGEIE are the interrupt-pending and
interrupt-enable bits for guest external interrupts at supervisor level
(HS-level).
SGEIP is read-only in hip, and is 1 if and only if the bitwise
logical-AND of CSRs hgeip and hgeie is nonzero in any bit.
(See Section 1.2.4.)
Bits hip.VSEIP and hie.VSEIE are the interrupt-pending and
interrupt-enable bits for VS-level external interrupts.
VSEIP is read-only in hip, and is the logical-OR of these interrupt
sources:
bit VSEIP of hvip;
the bit of hgeip selected by hstatus.VGEIN; and
any other platform-specific external interrupt signal directed to VS-level.
Bits hip.VSTIP and hie.VSTIE are the interrupt-pending and
interrupt-enable bits for VS-level timer interrupts.
VSTIP is read-only in hip, and is the logical-OR of
hvip.VSTIP and any other platform-specific timer interrupt signal
directed to VS-level.
Bits hip.VSSIP and hie.VSSIE are the interrupt-pending and
interrupt-enable bits for VS-level software interrupts.
VSSIP in hip is an alias (writable) of the same bit in hvip.
Multiple simultaneous interrupts destined for HS-mode are handled in the following decreasing priority order: SEI, SSI, STI, SGEI, VSEI, VSSI, VSTI.
5.2.4 Hypervisor Guest External Interrupt Registers (hgeip and hgeie)
The hgeip register is an HSXLEN-bit read-only register, formatted
as shown in Figure 1.12, that indicates pending guest external
interrupts for this hart.
The hgeie register is an HSXLEN-bit read/write register, formatted
as shown in Figure 1.13, that contains enable bits for the
guest external interrupts at this hart.
Guest external interrupt number i corresponds with
bit i in both hgeip and hgeie.
hgeip).
hgeie).Guest external interrupts represent interrupts directed to individual
virtual machines at VS-level.
If a RISC-V platform supports placing a physical device under the direct
control of a guest OS with minimal hypervisor intervention (known as
pass-through or direct assignment between a virtual machine
and the physical device), then, in such circumstance, interrupts from the
device are intended for a specific virtual machine.
Each bit of hgeip summarizes all pending interrupts directed
to one virtual hart, as collected and reported by an interrupt
controller.
To distinguish specific pending interrupts from multiple devices,
software must query the interrupt controller.
Support for guest external interrupts requires an interrupt controller that can collect virtual-machine-directed interrupts separately from other interrupts.
The number of bits implemented in hgeip and hgeie for guest
external interrupts is and may be zero.
This number is known as GEILEN.
The least-significant bits are implemented first, apart from bit 0.
Hence, if GEILEN is nonzero, bits GEILEN:1 shall be writable in
hgeie, and all other bit positions shall be read-only zeros in
both hgeip and hgeie.
The set of guest external interrupts received and handled at one physical hart may differ from those received at other harts. Guest external interrupt number i at one physical hart is typically expected not to be the same as guest external interrupt i at any other hart. For any one physical hart, the maximum number of virtual harts that may directly receive guest external interrupts is limited by GEILEN. The maximum this number can be for any implementation is 31 for RV32 and 63 for RV64, per physical hart.
A hypervisor is always free to emulate devices for any number of virtual harts without being limited by GEILEN. Only direct pass-through (direct assignment) of interrupts is affected by the GEILEN limit, and the limit is on the number of virtual harts receiving such interrupts, not the number of distinct interrupts received. The number of distinct interrupts a single virtual hart may receive is determined by the interrupt controller.
Register hgeie selects the subset of guest external interrupts that
cause a supervisor-level (HS-level) guest external interrupt.
The enable bits in hgeie do not affect the VS-level external
interrupt signal selected from hgeip by hstatus.VGEIN.
5.2.5
Hypervisor Environment Configuration Registers
(henvcfg and henvcfgh)
The henvcfg CSR is an HSXLEN-bit read/write register,
formatted for HSXLEN=64 as shown in Figure 1.14,
that controls certain
characteristics of the execution environment when virtualization mode
V=1.
henvcfg) for HSXLEN=64.If bit FIOM (Fence of I/O implies Memory) is set to one in
henvcfg, FENCE instructions executed when V=1 are modified
so the requirement to order accesses to device I/O implies also the
requirement to order main memory accesses.
Table [tab:henvcfg-FIOM] details the modified interpretation of
FENCE instruction bits PI, PO, SI, and SO when FIOM=1 and V=1.
Similarly, when FIOM=1 and V=1, if an atomic instruction that accesses a region ordered as device I/O has its aq and/or rl bit set, then that instruction is ordered as though it accesses both device I/O and memory.
| Instruction bit | Meaning when set |
|---|---|
| PI | Predecessor device input and memory reads (PR implied) |
| PO | Predecessor device output and memory writes (PW implied) |
| SI | Successor device input and memory reads (SR implied) |
| SO | Successor device output and memory writes (SW implied) |
The PBMTE bit controls whether the Svpbmt extension is available for use in VS-stage address translation. When PBMTE=1, Svpbmt is available for VS-stage address translation. When PBMTE=0, the implementation behaves as though Svpbmt were not implemented for VS-stage address translation. If Svpbmt is not implemented, PBMTE is read-only zero.
The definition of the VSTCE field will be furnished by the
forthcoming Sstc extension.
Its allocation within henvcfg may change prior to the ratification
of that extension.
The definition of the CBZE field will be furnished by the
forthcoming Zicboz extension.
Its allocation within henvcfg may change prior to the ratification
of that extension.
The definitions of the CBCFE and CBIE fields will be furnished by the
forthcoming Zicbom extension.
Their allocations within henvcfg may change prior to the ratification
of that extension.
When HSXLEN=32, henvcfg contains the same fields as bits 31:0
of henvcfg when HSXLEN=64.
Additionally, when HSXLEN=32, henvcfgh is a 32-bit read/write register that
contains the same fields as bits 63:32 of henvcfg when
HSXLEN=64.
Register henvcfgh does not exist when HSXLEN=64.
5.2.6 Hypervisor Counter-Enable Register (hcounteren)
The counter-enable register hcounteren is a 32-bit register that
controls the availability of the hardware performance monitoring counters
to the guest virtual machine.
hcounteren).When the CY, TM, IR, or HPMn bit in the hcounteren register
is clear, attempts to read the cycle, time, instret, or
hpmcountern register while V=1 will cause a virtual
instruction exception if the same bit in mcounteren is 1.
When one of these bits is set, access to the corresponding register is
permitted when V=1, unless prevented for some other reason.
In VU-mode, a counter is not readable unless the applicable bits are set
in both hcounteren and scounteren.
hcounteren must be implemented.
However, any of the bits may be read-only zero,
indicating reads to the corresponding counter will cause an exception
when V=1.
Hence, they are effectively WARL fields.
5.2.7 Hypervisor Time Delta Registers (htimedelta, htimedeltah)
The htimedelta CSR is a read/write register that contains the delta
between the value of the time CSR and the value returned in VS-mode or
VU-mode.
That is, reading the time CSR in VS or VU mode returns the sum of the
contents of htimedelta and the actual value of time.
Because overflow is ignored when summing htimedelta and time,
large values of htimedelta may be used to represent negative time
offsets.
For HSXLEN=32 only, htimedelta holds the lower 32 bits of the
delta, and htimedeltah holds the upper 32 bits of the delta.
5.2.8 Hypervisor Trap Value Register (htval)
The htval register is an HSXLEN-bit read/write register formatted
as shown in Figure 1.18.
When a trap is taken into HS-mode, htval is written with additional
exception-specific information, alongside stval, to assist software
in handling the trap.
htval).When a guest-page-fault trap is taken into HS-mode, htval is
written with either zero or the guest physical address that faulted,
shifted right by 2 bits.
For other traps, htval is set to zero, but a future standard or
extension may redefine htval’s setting for other traps.
A guest-page fault may arise due to an implicit memory access during
first-stage (VS-stage) address translation, in which case a guest
physical address written to htval is that of the implicit memory
access that faulted—for example, the address of a VS-level page table
entry that could not be read.
(The guest physical address corresponding to the original virtual address
is unknown when VS-stage translation fails to complete.)
Additional information is provided in CSR htinst to disambiguate
such situations.
Otherwise, for misaligned loads and stores that cause guest-page faults,
a nonzero guest physical address in htval corresponds to the
faulting portion of the access as indicated by the virtual address in
stval.
For instruction guest-page faults on systems with variable-length
instructions, a nonzero htval corresponds to the faulting portion
of the instruction as indicated by the virtual address in stval.
A guest physical address written to htval is shifted right by
2 bits to accommodate addresses wider than the current XLEN.
For RV32, the hypervisor extension permits guest physical addresses as
wide as 34 bits, and htval reports bits 33:2 of the address.
This shift-by-2 encoding of guest physical addresses matches the encoding
of physical addresses in PMP address registers (Section [sec:pmp])
and in page table entries (Sections [sec:sv32], [sec:sv39],
[sec:sv48], and [sec:sv57]).
If the least-significant two bits of a faulting guest physical address
are needed, these bits are ordinarily the same as the least-significant
two bits of the faulting virtual address in stval.
For faults due to implicit memory accesses for VS-stage address
translation, the least-significant two bits are instead zeros.
These cases can be distinguished using the value provided in register
htinst.
htval is a WARL register that must be able to hold zero and may
be capable of holding only an arbitrary subset of other 2-bit-shifted
guest physical addresses, if any.
Unless it has reason to assume otherwise (such as a platform standard),
software that writes a value to htval should read back from
htval to confirm the stored value.
5.2.9 Hypervisor Trap Instruction Register (htinst)
The htinst register is an HSXLEN-bit read/write register formatted
as shown in Figure 1.19.
When a trap is taken into HS-mode, htinst is written with a value
that, if nonzero, provides information about the instruction that
trapped, to assist software in handling the trap.
The values that may be written to htinst on a trap are documented
in Section 1.6.3.
htinst).htinst is a WARL register that need only be able to hold the
values that the implementation may automatically write to it on a trap.
5.2.10 Hypervisor Guest Address Translation and Protection Register (hgatp)
The hgatp register is an HSXLEN-bit read/write register, formatted as
shown in Figure 1.20 for HSXLEN=32 and Figure 1.21 for
HSXLEN=64, which controls G-stage address translation and protection, the
second stage of two-stage translation for guest virtual addresses (see
Section 1.5).
Similar to CSR satp, this register holds the physical page number (PPN)
of the guest-physical root page table; a virtual machine identifier (VMID),
which facilitates address-translation fences on a per-virtual-machine basis;
and the MODE field, which selects the address-translation scheme for guest
physical addresses.
When mstatus.TVM=1, attempts to read or write hgatp while executing
in HS-mode will raise an illegal instruction exception.
hgatp when HSXLEN=32.
hgatp when HSXLEN=64, for MODE values Bare, Sv39x4, Sv48x4, and Sv57x4.Table 1.22 shows the encodings of the MODE field when HSXLEN=32 and
HSXLEN=64.
When MODE=Bare, guest physical addresses are equal to supervisor physical
addresses, and there is no further memory protection for a guest virtual
machine beyond the physical memory protection scheme described in
Section [sec:pmp].
In this case, the remaining fields in hgatp must be set to zeros.
When HSXLEN=32, the only other valid setting for MODE is Sv32x4, which is a modification of the usual Sv32 paged virtual-memory scheme, extended to support 34-bit guest physical addresses. When HSXLEN=64, modes Sv39x4, Sv48x4, and Sv57x4 are defined as modifications of the Sv39, Sv48, and Sv57 paged virtual-memory schemes. All of these paged virtual-memory schemes are described in Section 1.5.1.
The remaining MODE settings when HSXLEN=64 are reserved for future use and may define
different interpretations of the other fields in hgatp.
hgatp MODE field.Implementations are not required to support all defined MODE settings when HSXLEN=64.
A write to hgatp with an unsupported MODE value is not ignored as it is
for satp.
Instead, the fields of hgatp are WARL in the normal way, when so
indicated.
As explained in Section 1.5.1, for the paged
virtual-memory schemes (Sv32x4, Sv39x4, Sv48x4, and Sv57x4), the root page table is
16 KiB and must be aligned to a 16-KiB boundary.
In these modes, the lowest two bits of the physical page number (PPN) in
hgatp always read as zeros.
An implementation that supports only the defined paged virtual-memory schemes
and/or Bare may make PPN[1:0] read-only zero.
The number of VMID bits is and may be zero.
The number of implemented VMID bits, termed VMIDLEN , may be
determined by writing one to every bit position in the VMID field, then reading
back the value in hgatp to see which bit positions in the VMID field hold
a one.
The least-significant bits of VMID are implemented first:
that is, if VMIDLEN > 0, VMID[VMIDLEN-1:0] is writable.
The maximal value of VMIDLEN, termed VMIDMAX, is 7 for Sv32x4 or 14 for Sv39x4,
Sv48x4, and Sv57x4.
The hgatp register is considered active for the purposes of the
address-translation algorithm unless the effective privilege mode is U
and hstatus.HU=0.
This definition simplifies the implementation of speculative execution of HLV, HLVX, and HSV instructions.
Note that writing hgatp does not imply any ordering constraints between
page-table updates and subsequent G-stage address translations.
If the new virtual machine’s guest physical page tables have been modified,
or if a VMID is reused,
it may be necessary to execute an HFENCE.GVMA instruction
(see Section 1.3.2) before or after writing hgatp.
5.2.11 Virtual Supervisor Status Register (vsstatus)
The vsstatus register is a VSXLEN-bit read/write register that is
VS-mode’s version of supervisor register sstatus, formatted as
shown in Figure 1.23 when VSXLEN=32 and
Figure 1.24 when VSXLEN=64.
When V=1, vsstatus substitutes for the usual sstatus, so
instructions that normally read or modify sstatus actually access
vsstatus instead.
vsstatus) when VSXLEN=32.
vsstatus) when VSXLEN=64.The UXL field controls the effective XLEN for VU-mode, which may differ
from the XLEN for VS-mode (VSXLEN).
When VSXLEN=32, the UXL field does not exist, and VU-mode XLEN=32.
When VSXLEN=64, UXL is a WARL field that is encoded the same as the MXL
field of misa, shown in Table [misabase] on
page .
In particular, an implementation may make UXL be a read-only copy of
field VSXL of hstatus, forcing VU-mode XLEN=VSXLEN.
If VSXLEN is changed from 32 to a wider width, and if field UXL is not restricted to a single value, it gets the value corresponding to the widest supported width not wider than the new VSXLEN.
When V=1, both vsstatus.FS and the HS-level sstatus.FS are in
effect. Attempts
to execute a floating-point instruction when either field is 0 (Off) raise an
illegal-instruction exception. Modifying the floating-point state when V=1
causes both fields to be set to 3 (Dirty).
For a hypervisor to benefit from the extension context status, it must
have its own copy in the HS-level sstatus, maintained independently
of a guest OS running in VS-mode.
While a version of the extension context status obviously must exist in
vsstatus for VS-mode, a hypervisor cannot rely on this version
being maintained correctly, given that VS-level software can change
vsstatus.FS arbitrarily.
If the HS-level sstatus.FS were not independently active and
maintained by the hardware in parallel with vsstatus.FS while V=1,
hypervisors would always be forced to conservatively swap all
floating-point state when context-switching between virtual machines.
Similarly, when V=1, both vsstatus.VS and the HS-level sstatus.VS
are in effect.
Attempts to execute a vector instruction when either field is 0 (Off) raise an
illegal-instruction exception.
Modifying the vector state when V=1 causes both fields to be set to 3 (Dirty).
Read-only fields SD and XS summarize the extension context status as it
is visible to VS-mode only.
For example, the value of the HS-level sstatus.FS does not affect
vsstatus.SD.
An implementation may make field UBE be a read-only copy of
hstatus.VSBE.
When V=0, vsstatus does not directly affect the behavior of the machine,
unless a virtual-machine load/store (HLV, HLVX, or HSV)
or the MPRV feature in the mstatus
register is used to execute a load or store
as though V=1.
5.2.12 Virtual Supervisor Interrupt Registers (vsip and vsie)
The vsip and vsie registers are VSXLEN-bit read/write
registers that are VS-mode’s versions of supervisor CSRs sip and
sie, formatted as shown in Figures 1.25 and 1.26
respectively.
When V=1, vsip and vsie substitute for the usual sip
and sie, so instructions that normally read or modify
sip/sie actually access vsip/vsie instead.
However, interrupts directed to HS-level continue to be
indicated in the HS-level sip register, not in vsip, when
V=1.
vsip).
vsie).The standard portions (bits 15:0) of registers vsip and vsie
are formatted as shown in Figures 1.27 and
1.28 respectively.
vsip.
vsie.When bit 10 of hideleg is zero, vsip.SEIP and vsie.SEIE
are read-only zeros.
Else, vsip.SEIP and vsie.SEIE are aliases of hip.VSEIP
and hie.VSEIE.
When bit 6 of hideleg is zero, vsip.STIP and vsie.STIE
are read-only zeros.
Else, vsip.STIP and vsie.STIE are aliases of hip.VSTIP
and hie.VSTIE.
When bit 2 of hideleg is zero, vsip.SSIP and vsie.SSIE
are read-only zeros.
Else, vsip.SSIP and vsie.SSIE are aliases of hip.VSSIP
and hie.VSSIE.
5.2.13 Virtual Supervisor Trap Vector Base Address Register (vstvec)
The vstvec register is a VSXLEN-bit read/write register that is
VS-mode’s version of supervisor register stvec, formatted as shown
in Figure 1.29.
When V=1, vstvec substitutes for the usual stvec, so
instructions that normally read or modify stvec actually access
vstvec instead.
When V=0, vstvec does not directly affect the behavior of the
machine.
vstvec).5.2.14 Virtual Supervisor Scratch Register (vsscratch)
The vsscratch register is a VSXLEN-bit read/write register that is
VS-mode’s version of supervisor register sscratch, formatted as
shown in Figure 1.30.
When V=1, vsscratch substitutes for the usual sscratch, so
instructions that normally read or modify sscratch actually access
vsscratch instead.
The contents of vsscratch never directly affect the behavior of
the machine.
vsscratch).5.2.15 Virtual Supervisor Exception Program Counter (vsepc)
The vsepc register is a VSXLEN-bit read/write register that is
VS-mode’s version of supervisor register sepc, formatted as shown
in Figure 1.31.
When V=1, vsepc substitutes for the usual sepc, so
instructions that normally read or modify sepc actually access
vsepc instead.
When V=0, vsepc does not directly affect the behavior of the
machine.
vsepc is a WARL register that must be able to hold the same set of
values that sepc can hold.
vsepc).5.2.16 Virtual Supervisor Cause Register (vscause)
The vscause register is a VSXLEN-bit read/write register that is
VS-mode’s version of supervisor register scause, formatted as shown
in Figure 1.32.
When V=1, vscause substitutes for the usual scause, so
instructions that normally read or modify scause actually access
vscause instead.
When V=0, vscause does not directly affect the behavior of the
machine.
vscause is a WLRL register that must be able to hold the same set of
values that scause can hold.
vscause).5.2.17 Virtual Supervisor Trap Value Register (vstval)
The vstval register is a VSXLEN-bit read/write register that is
VS-mode’s version of supervisor register stval, formatted as shown
in Figure 1.33.
When V=1, vstval substitutes for the usual stval, so
instructions that normally read or modify stval actually access
vstval instead.
When V=0, vstval does not directly affect the behavior of the
machine.
vstval is a WARL register that must be able to hold the same set of
values that stval can hold.
vstval).5.2.18 Virtual Supervisor Address Translation and Protection Register (vsatp)
The vsatp register is a VSXLEN-bit read/write register that is
VS-mode’s version of supervisor register satp, formatted as shown
in Figure 1.34 for VSXLEN=32 and Figure 1.35
for VSXLEN=64.
When V=1, vsatp substitutes for the usual satp, so
instructions that normally read or modify satp actually access
vsatp instead.
vsatp controls VS-stage address translation, the first stage of
two-stage translation for guest virtual addresses (see
Section 1.5).
vsatp when VSXLEN=32.
vsatp when VSXLEN=64, for MODE
values Bare, Sv39, Sv48, and Sv57.The vsatp register is considered active for the purposes of the
address-translation algorithm unless the effective privilege mode is U
and hstatus.HU=0.
However, even when vsatp is active, VS-stage page-table entries’ A bits
must not be set as a result of speculative execution, unless the effective
privilege mode is VS or VU.
In particular, virtual-machine load/store (HLV, HLVX, or HSV) instructions that are misspeculatively executed must not cause VS-stage A bits to be set.
When V=0, a write to vsatp with an unsupported MODE value is not
ignored as it is for satp.
Instead, the fields of vsatp are WARL in the normal way.
When V=0, vsatp does not directly affect the behavior of the machine,
unless a virtual-machine load/store (HLV, HLVX, or HSV)
or the MPRV feature in the mstatus
register is used to execute a load or store
as though V=1.
5.3 Hypervisor Instructions
The hypervisor extension adds virtual-machine load and store instructions and two privileged fence instructions.
5.3.1 Hypervisor Virtual-Machine Load and Store Instructions
The hypervisor virtual-machine load and store instructions are valid only
in M-mode or HS-mode, or in U-mode when hstatus.HU=1.
Each instruction performs an explicit memory access as though V=1;
i.e., with the address translation and protection, and the endianness,
that apply to memory accesses in either VS-mode or VU-mode.
Field SPVP of hstatus controls the privilege level of the access.
The explicit memory access is done as though in VU-mode when SPVP=0, and
as though in VS-mode when SPVP=1.
As usual when V=1, two-stage address translation is applied, and the
HS-level sstatus.SUM is ignored.
HS-level sstatus.MXR makes execute-only pages readable for
both stages of address translation (VS-stage and G-stage), whereas
vsstatus.MXR affects only the first translation stage (VS-stage).
For every RV32I or RV64I load instruction, LB, LBU, LH, LHU, LW, LWU, and LD, there is a corresponding virtual-machine load instruction: HLV.B, HLV.BU, HLV.H, HLV.HU, HLV.W, HLV.WU, and HLV.D. For every RV32I or RV64I store instruction, SB, SH, SW, and SD, there is a corresponding virtual-machine store instruction: HSV.B, HSV.H, HSV.W, and HSV.D. Instructions HLV.WU, HLV.D, and HSV.D are not valid for RV32, of course.
Instructions HLVX.HU and HLVX.WU are the same as HLV.HU and HLV.WU, except that execute permission takes the place of read permission during address translation. That is, the memory being read must be executable in both stages of address translation, but read permission is not required. For the supervisor physical address that results from address translation, the supervisor physical memory attributes must grant both execute and read permissions. (The supervisor physical memory attributes are the machine’s physical memory attributes as modified by physical memory protection, Section [sec:pmp], for supervisor level.)
HLVX cannot override machine-level physical memory protection (PMP), so attempting to read memory that PMP designates as execute-only still results in an access-fault exception.
HLVX.WU is valid for RV32, even though LWU and HLV.WU are not. (For RV32, HLVX.WU can be considered a variant of HLV.W, as sign extension is irrelevant for 32-bit values.)
Attempts to execute a virtual-machine load/store instruction (HLV, HLVX,
or HSV) when V=1 cause a virtual instruction trap.
Attempts to execute one of these same instructions from U-mode when
hstatus.HU=0 cause an illegal instruction trap.
5.3.2 Hypervisor Memory-Management Fence Instructions
The hypervisor memory-management fence instructions, HFENCE.VVMA
and HFENCE.GVMA, perform a function similar to SFENCE.VMA
(Section [sec:sfence.vma]), except applying to the VS-level
memory-management data structures controlled by CSR vsatp
(HFENCE.VVMA) or the guest-physical memory-management data structures
controlled by CSR hgatp (HFENCE.GVMA).
Instruction SFENCE.VMA applies only to the memory-management data structures
controlled by the current satp (either the HS-level satp when
V=0 or vsatp when V=1).
HFENCE.VVMA is valid only in M-mode or HS-mode. Its effect is much the same as temporarily entering VS-mode and executing SFENCE.VMA. Executing an HFENCE.VVMA guarantees that any previous stores already visible to the current hart are ordered before all subsequent implicit reads by that hart of the VS-level memory-management data structures, when those implicit reads are for instructions that
are subsequent to the HFENCE.VVMA, and
execute when hgatp.VMID has the same setting as it did when HFENCE.VVMA
executed.
Implicit reads need not be ordered when hgatp.VMID is different than at
the time HFENCE.VVMA executed.
If operand rs1≠x0, it specifies a single guest virtual
address, and if operand rs2≠x0, it specifies a single guest
address-space identifier
(ASID).
An HFENCE.VVMA instruction applies only to a single virtual machine, identified
by the setting of hgatp.VMID when HFENCE.VVMA executes.
When rs2≠x0, bits XLEN-1:ASIDMAX of the value held in
rs2 are reserved for future standard use. Until their use is defined by a
standard extension, they should be zeroed by software and ignored
by current implementations.
Furthermore, if ASIDLEN < ASIDMAX, the implementation shall ignore bits
ASIDMAX-1:ASIDLEN of the value held in rs2.
Simpler implementations of HFENCE.VVMA can ignore the guest virtual address in
rs1 and the guest ASID value in rs2, as well as hgatp.VMID,
and always perform a global fence for the VS-level memory management of all
virtual machines, or even a global fence for all memory-management data
structures.
Neither mstatus.TVM nor hstatus.VTVM causes HFENCE.VVMA to
trap.
HFENCE.GVMA is valid only in HS-mode when mstatus.TVM=0, or in
M-mode (irrespective of mstatus.TVM).
Executing an HFENCE.GVMA instruction guarantees that any previous stores
already visible to the current hart are ordered before all subsequent implicit
reads by that hart of guest-physical memory-management data structures done for instructions
that follow the HFENCE.GVMA.
If operand rs1≠x0, it specifies a single guest physical
address, shifted right by 2 bits, and if operand rs2≠x0, it
specifies a single virtual machine identifier (VMID).
Like for a guest physical address written to htval on a
trap, a guest physical address specified in rs1 is shifted
right by 2 bits to accommodate addresses wider than the current XLEN.
When rs2≠x0, bits XLEN-1:VMIDMAX of the value held in
rs2 are reserved for future standard use. Until their use is defined by a
standard extension, they should be zeroed by software and ignored
by current implementations.
Furthermore, if VMIDLEN < VMIDMAX, the implementation shall ignore bits
VMIDMAX-1:VMIDLEN of the value held in rs2.
Simpler implementations of HFENCE.GVMA can ignore the guest physical address in rs1 and the VMID value in rs2 and always perform a global fence for the guest-physical memory management of all virtual machines, or even a global fence for all memory-management data structures.
If hgatp.MODE is changed for a given VMID, an HFENCE.GVMA with
rs1=x0 (and rs2 set to either x0 or the VMID) must
be executed to order subsequent guest translations with the MODE
change—even if the old MODE or new MODE is Bare.
Attempts to execute HFENCE.VVMA or HFENCE.GVMA when V=1 cause a virtual
instruction trap, while attempts to do the same in U-mode
cause an illegal instruction trap.
Attempting to execute HFENCE.GVMA in HS-mode when mstatus.TVM=1
also causes an illegal instruction trap.
5.4 Machine-Level CSRs
The hypervisor extension augments or modifies machine CSRs mstatus,
mstatush, mideleg, mip, and mie, and
adds CSRs mtval2 and mtinst.
5.4.1 Machine Status Registers (mstatus and mstatush)
The hypervisor extension adds two fields, MPV and GVA, to the
machine-level mstatus or mstatush CSR, and modifies the
behavior of several existing mstatus fields.
Figure 1.36 shows the modified mstatus register
when the hypervisor extension is implemented and MXLEN=64.
When MXLEN=32, the hypervisor extension adds MPV and GVA not to mstatus
but to mstatush.
Figure 1.37 shows the mstatush register when
the hypervisor extension is implemented and MXLEN=32.
mstatus) for RV64 when the hypervisor extension is implemented.
mstatush) for RV32 when the hypervisor extension is implemented.
The format of mstatus is unchanged for RV32.The MPV bit (Machine Previous Virtualization Mode) is written by the implementation whenever a trap is taken into M-mode. Just as the MPP field is set to the (nominal) privilege mode at the time of the trap, the MPV bit is set to the value of the virtualization mode V at the time of the trap. When an MRET instruction is executed, the virtualization mode V is set to MPV, unless MPP=3, in which case V remains 0.
Field GVA (Guest Virtual Address) is written by the implementation
whenever a trap is taken into M-mode.
For any trap (breakpoint, address misaligned,
access fault, page fault, or guest-page fault) that writes
a guest virtual address to mtval, GVA is set to 1.
For any other trap into M-mode, GVA is set to 0.
The TSR and TVM fields of mstatus affect execution only in HS-mode,
not in VS-mode.
The TW field affects execution in all modes except M-mode.
Setting TVM=1 prevents HS-mode from accessing hgatp or executing
HFENCE.GVMA or HINVAL.GVMA, but has no effect on accesses to vsatp or
instructions HFENCE.VVMA or HINVAL.VVMA.
TVM exists in mstatus to allow machine-level software to modify
the address translations managed by a supervisor-level OS, usually for
the purpose of inserting another stage of address translation below
that controlled by the OS.
The instruction traps enabled by TVM=1 permit machine level
to co-opt both satp and hgatp and substitute
shadow page tables that merge the OS’s chosen page translations
with M-level’s lower-stage translations, all without the OS being
aware.
M-level software needs this ability not only to emulate the hypervisor
extension if not already supported, but also to emulate any future
RISC-V extensions that may modify or add address translation
stages, perhaps, for example, to improve support for nested
hypervisors, i.e., running hypervisors atop other hypervisors.
However, setting TVM=1 does not cause traps for accesses to vsatp
or instructions HFENCE.VVMA or HINVAL.VVMA, or for any actions taken
in VS-mode, because M-level software is not expected to need to involve
itself in VS-stage address translation.
For virtual machines, it should be sufficient, and in all likelihood
faster as well, to leave VS-stage address translation alone and merge
all other translation stages into G-stage shadow page tables controlled
by hgatp.
This assumption does place some constraints on possible future
RISC-V extensions that current machines will be able to emulate
efficiently.
The hypervisor extension changes the behavior of the the Modify Privilege
field, MPRV, of mstatus.
When MPRV=0, translation and protection behave as normal.
When MPRV=1, explicit memory accesses are translated and protected, and
endianness is applied, as though the current virtualization mode were set
to MPV and the current nominal privilege mode were set to MPP.
Table [h-mprv] enumerates the cases.
| MPRV | MPV | MPP | Effect |
|---|---|---|---|
| 0 | – | – | Normal access; current privilege mode applies. |
| 1 | 0 | 0 | U-level access with HS-level translation and protection only. |
| 1 | 0 | 1 | HS-level access with HS-level translation and protection only. |
| 1 | – | 3 | M-level access with no translation. |
| 1 | 1 | 0 | VU-level access with two-stage translation and protection. The HS-level MXR bit makes any executable page readable. vsstatus.MXR makes readable those pages marked executable at the VS translation stage, but only if readable at the guest-physical translation stage. |
| 1 | 1 | 1 | VS-level access with two-stage translation and protection. The HS-level MXR bit makes any executable page readable. vsstatus.MXR makes readable those pages marked executable at the VS translation stage, but only if readable at the guest-physical translation stage. vsstatus.SUM applies instead of the HS-level SUM bit. |
MPRV does not affect the virtual-machine load/store instructions, HLV,
HLVX, and HSV.
The explicit loads and stores of these instructions always act as though
V=1 and the nominal privilege mode were hstatus.SPVP, overriding MPRV.
The mstatus register is a superset of the HS-level sstatus
register but is not a superset of vsstatus.
5.4.2 Machine Interrupt Delegation Register (mideleg)
When the hypervisor extension is implemented, bits 10, 6, and 2 of
mideleg (corresponding to the standard VS-level interrupts) are
each read-only one.
Furthermore, if any guest external interrupts are implemented (GEILEN is
nonzero), bit 12 of mideleg (corresponding to supervisor-level
guest external interrupts) is also read-only one.
VS-level interrupts and guest external interrupts are always delegated
past M-mode to HS-mode.
For bits of mideleg that are zero, the corresponding bits in
hideleg, hip, and hie are read-only zeros.
5.4.3 Machine Interrupt Registers (mip and mie)
The hypervisor extension gives registers mip and mie
additional active bits for the hypervisor-added interrupts.
Figures 1.38 and
1.39 show the standard portions (bits 15:0)
of registers mip and mie when the hypervisor extension is
implemented.
mip.
mie.Bits SGEIP, VSEIP, VSTIP, and VSSIP in mip are aliases for the same bits
in hypervisor CSR hip, while SGEIE, VSEIE, VSTIE, and VSSIE in mie
are aliases for the same bits in hie.
5.4.4 Machine Second Trap Value Register (mtval2)
The mtval2 register is an MXLEN-bit read/write register formatted
as shown in Figure 1.40.
When a trap is taken into M-mode, mtval2 is written with additional
exception-specific information, alongside mtval, to assist software
in handling the trap.
mtval2).When a guest-page-fault trap is taken into M-mode, mtval2 is
written with either zero or the guest physical address that faulted,
shifted right by 2 bits.
For other traps, mtval2 is set to zero, but a future standard or
extension may redefine mtval2’s setting for other traps.
If a guest-page fault is due to an implicit memory access during
first-stage (VS-stage) address translation, a guest physical address
written to mtval2 is that of the implicit memory access that
faulted.
Additional information is provided in CSR mtinst to disambiguate
such situations.
Otherwise, for misaligned loads and stores that cause guest-page faults,
a nonzero guest physical address in mtval2 corresponds to the
faulting portion of the access as indicated by the virtual address in
mtval.
For instruction guest-page faults on systems with variable-length
instructions, a nonzero mtval2 corresponds to the faulting portion
of the instruction as indicated by the virtual address in mtval.
mtval2 is a WARL register that must be able to hold zero and may
be capable of holding only an arbitrary subset of other 2-bit-shifted
guest physical addresses, if any.
5.4.5 Machine Trap Instruction Register (mtinst)
The mtinst register is an MXLEN-bit read/write register formatted
as shown in Figure 1.41.
When a trap is taken into M-mode, mtinst is written with a value
that, if nonzero, provides information about the instruction that
trapped, to assist software in handling the trap.
The values that may be written to mtinst on a trap are documented
in Section 1.6.3.
mtinst).mtinst is a WARL register that need only be able to hold the
values that the implementation may automatically write to it on a trap.
5.5 Two-Stage Address Translation
Whenever the current virtualization mode V is 1,
two-stage address translation and protection is in
effect.
For any virtual memory access, the original virtual address is
converted in the first stage
by VS-level address translation, as controlled by the vsatp
register, into a guest physical address.
The guest physical address is then converted
in the second stage by guest physical address
translation, as controlled by the hgatp register, into a supervisor
physical address.
The two stages are known also as VS-stage and G-stage translation.
Although there is no option to disable two-stage address translation when V=1,
either stage of translation can be effectively disabled by zeroing the
corresponding vsatp or hgatp register.
The vsstatus field MXR, which makes execute-only pages readable, only
overrides VS-stage page protection.
Setting MXR at VS-level does not override guest-physical page protections.
Setting MXR at HS-level, however, overrides both VS-stage and G-stage
execute-only permissions.
When V=1, memory accesses that would normally bypass address translation are subject to G-stage address translation alone. This includes memory accesses made in support of VS-stage address translation, such as reads and writes of VS-level page tables.
Machine-level physical memory protection applies to supervisor physical addresses and is in effect regardless of virtualization mode.
5.5.1 Guest Physical Address Translation
The mapping of guest physical addresses to supervisor physical addresses is
controlled by CSR hgatp (Section 1.2.10).
When the address translation scheme selected by the MODE field of hgatp
is Bare, guest physical addresses are equal to supervisor physical addresses
without modification, and no memory protection applies in the trivial
translation of guest physical addresses to supervisor physical addresses.
When hgatp.MODE specifies a translation scheme of Sv32x4, Sv39x4,
Sv48x4, or Sv57x4, G-stage address translation is a variation on the usual
page-based virtual address translation scheme of Sv32, Sv39, Sv48, or Sv57,
respectively.
In each case, the size of the incoming address is widened by 2 bits (to 34, 41,
or 50 bits).
To accommodate the 2 extra bits, the root page table (only) is expanded by a
factor of four to be 16 KiB instead of the usual 4 KiB.
Matching its larger size, the root page table also must be aligned to a 16 KiB
boundary instead of the usual 4 KiB page boundary.
Except as noted, all other aspects of Sv32, Sv39, Sv48, or Sv57 are adopted
unchanged for G-stage translation.
Non-root page tables and all page table entries (PTEs) have the same formats as
documented in Sections [sec:sv32], [sec:sv39], [sec:sv48],
and [sec:sv57].
For Sv32x4, an incoming guest physical address is partitioned into a virtual page number (VPN) and page offset as shown in Figure 1.42. This partitioning is identical to that for an Sv32 virtual address as depicted in Figure [sv32va] (page ), except with 2 more bits at the high end in VPN[1]. (Note that the fields of a partitioned guest physical address also correspond one-for-one with the structure that Sv32 assigns to a physical address, depicted in Figure [rv32va].)
For Sv39x4, an incoming guest physical address is partitioned as shown in Figure 1.43. This partitioning is identical to that for an Sv39 virtual address as depicted in Figure [sv39va] (page ), except with 2 more bits at the high end in VPN[2]. Address bits 63:41 must all be zeros, or else a guest-page-fault exception occurs.
For Sv48x4, an incoming guest physical address is partitioned as shown in Figure 1.44. This partitioning is identical to that for an Sv48 virtual address as depicted in Figure [sv48va] (page ), except with 2 more bits at the high end in VPN[3]. Address bits 63:50 must all be zeros, or else a guest-page-fault exception occurs.
For Sv57x4, an incoming guest physical address is partitioned as shown in Figure 1.45. This partitioning is identical to that for an Sv57 virtual address as depicted in Figure [sv57va] (page ), except with 2 more bits at the high end in VPN[3]. Address bits 63:50 must all be zeros, or else a guest-page-fault exception occurs.
The page-based G-stage address translation scheme for RV32, Sv32x4, is defined to support a 34-bit guest physical address so that an RV32 hypervisor need not be limited in its ability to virtualize real 32-bit RISC-V machines, even those with 33-bit or 34-bit physical addresses. This may include the possibility of a machine virtualizing itself, if it happens to use 33-bit or 34-bit physical addresses. Multiplying the size and alignment of the root page table by a factor of four is the cheapest way to extend Sv32 to cover a 34-bit address. The possible wastage of 12 KiB for an unnecessarily large root page table is expected to be of negligible consequence for most (maybe all) real uses.
A consistent ability to virtualize machines having as much as four times the physical address space as virtual address space is believed to be of some utility also for RV64. For a machine implementing 39-bit virtual addresses (Sv39), for example, this allows the hypervisor extension to support up to a 41-bit guest physical address space without either necessitating hardware support for 48-bit virtual addresses (Sv48) or falling back to emulating the larger address space using shadow page tables.
The conversion of an Sv32x4, Sv39x4, Sv48x4, or Sv57x4 guest physical address is accomplished with the same algorithm used for Sv32, Sv39, Sv48, or Sv57, as presented in Section [sv32algorithm], except that:
hgatp substitutes for the usual satp;
for the translation to begin, the effective privilege mode must be VS-mode or VU-mode;
when checking the U bit, the current privilege mode is always taken to be U-mode; and
guest-page-fault exceptions are raised instead of regular page-fault exceptions.
For G-stage address translation, all memory accesses (including those made to access data structures for VS-stage address translation) are considered to be user-level accesses, as though executed in U-mode. Access type permissions—readable, writable, or executable—are checked during G-stage translation the same as for VS-stage translation. For a memory access made to support VS-stage address translation (such as to read/write a VS-level page table), permissions are checked as though for a load or store, not for the original access type. However, any exception is always reported for the original access type (instruction, load, or store/AMO).
The G bit in all G-stage PTEs is reserved for future standard use. Until its use is defined by a standard extension, it should be cleared by software for forward compatibility, and must be ignored by hardware.
G-stage address translation uses the identical format for PTEs as regular address translation, even including the U bit, due to the possibility of sharing some (or all) page tables between G-stage translation and regular HS-level address translation. Regardless of whether this usage will ever become common, we chose not to preclude it.
5.5.2 Guest-Page Faults
Guest-page-fault traps may be delegated from M-mode to HS-mode under the
control of CSR medeleg, but cannot be delegated to other privilege
modes.
On a guest-page fault, CSR mtval or stval is written with the
faulting guest virtual address as usual, and mtval2 or htval is
written either with zero or with the faulting guest physical address,
shifted right by 2 bits.
CSR mtinst or htinst may also be written with information
about the faulting instruction or other reason for the access, as
explained in Section 1.6.3.
When an instruction fetch or a misaligned memory access straddles a page
boundary, two different address translations are involved.
When a guest-page fault occurs in such a circumstance, the faulting
virtual address written to mtval/stval is the same as would
be required for a regular page fault.
Thus, the faulting virtual address may be a page-boundary address that is
higher than the instruction’s original virtual address, if the byte at
that page boundary is among the accessed bytes.
When a guest-page fault is not due to an implicit
memory access for VS-stage address translation,
a nonzero guest physical address written to
mtval2/htval shall correspond
to the exact virtual address written to
mtval/stval.
5.5.3 Memory-Management Fences
The behavior of the SFENCE.VMA instruction is affected by the current virtualization mode V. When V=0, the virtual-address argument is an HS-level virtual address, and the ASID argument is an HS-level ASID. The instruction orders stores only to HS-level address-translation structures with subsequent HS-level address translations.
When V=1, the virtual-address argument to SFENCE.VMA is a guest virtual
address within the current virtual machine, and the ASID argument is a VS-level
ASID within the current virtual machine.
The current virtual machine is identified by the VMID field of CSR hgatp,
and the effective ASID can be considered to be the combination of this VMID
with the VS-level ASID.
The SFENCE.VMA instruction orders stores only to the VS-level
address-translation structures with subsequent VS-stage address translations
for the same virtual machine, i.e., only when hgatp.VMID is the same as
when the SFENCE.VMA executed.
Hypervisor instructions HFENCE.VVMA and HFENCE.GVMA provide additional memory-management fences to complement SFENCE.VMA. These instructions are described in Section 1.3.2.
Section [pmp-vmem] discusses the intersection between physical memory
protection (PMP) and page-based address translation.
It is noted there that, when PMP settings are modified in a manner that affects
either the physical memory that holds page tables or the physical memory to
which page tables point, M-mode software must synchronize the PMP settings with
the virtual memory system.
For HS-level address translation, this is accomplished by executing in M-mode
an SFENCE.VMA instruction with rs1=x0 and rs2=x0, after
the PMP CSRs are written.
If G-stage address translation is in use and is not Bare,
synchronization with its data
structures is also needed.
When PMP settings are modified in a manner that affects either the physical
memory that holds guest-physical page tables or the physical memory to which
guest-physical page tables point, an HFENCE.GVMA instruction with
rs1=x0 and rs2=x0 must be executed in M-mode after the
PMP CSRs are written.
An HFENCE.VVMA instruction is not required.
5.6 Traps
5.6.1 Trap Cause Codes
The hypervisor extension augments the trap cause encoding. Table [hcauses] lists the possible M-mode and HS-mode trap cause codes when the hypervisor extension is implemented. Codes are added for VS-level interrupts (interrupts 2, 6, 10), for supervisor-level guest external interrupts (interrupt 12), for virtual instruction exceptions (exception 22), and for guest-page faults (exceptions 20, 21, 23). Furthermore, environment calls from VS-mode are assigned cause 10, whereas those from HS-mode or S-mode use cause 9 as usual.
| Interrupt | Exception Code | Description | |
|---|---|---|---|
| 1 | 0 | Reserved | |
| 1 | 1 | Supervisor software interrupt | |
| 1 | 2 | Virtual supervisor software interrupt | |
| 1 | 3 | Machine software interrupt | |
| 1 | 4 | Reserved | |
| 1 | 5 | Supervisor timer interrupt | |
| 1 | 6 | Virtual supervisor timer interrupt | |
| 1 | 7 | Machine timer interrupt | |
| 1 | 8 | Reserved | |
| 1 | 9 | Supervisor external interrupt | |
| 1 | 10 | Virtual supervisor external interrupt | |
| 1 | 11 | Machine external interrupt | |
| 1 | 12 | Supervisor guest external interrupt | |
| 1 | 13–15 | Reserved | |
| 1 | ≥16 | Designated for platform or custom use | |
| 0 | 0 | Instruction address misaligned | |
| 0 | 1 | Instruction access fault | |
| 0 | 2 | Illegal instruction | |
| 0 | 3 | Breakpoint | |
| 0 | 4 | Load address misaligned | |
| 0 | 5 | Load access fault | |
| 0 | 6 | Store/AMO address misaligned | |
| 0 | 7 | Store/AMO access fault | |
| 0 | 8 | Environment call from U-mode or VU-mode | |
| 0 | 9 | Environment call from HS-mode | |
| 0 | 10 | Environment call from VS-mode | |
| 0 | 11 | Environment call from M-mode | |
| 0 | 12 | Instruction page fault | |
| 0 | 13 | Load page fault | |
| 0 | 14 | Reserved | |
| 0 | 15 | Store/AMO page fault | |
| 0 | 16–19 | Reserved | |
| 0 | 20 | Instruction guest-page fault | |
| 0 | 21 | Load guest-page fault | |
| 0 | 22 | Virtual instruction | |
| 0 | 23 | Store/AMO guest-page fault | |
| 0 | 24–31 | Designated for custom use | |
| 0 | 32–47 | Reserved | |
| 0 | 48–63 | Designated for custom use | |
| 0 | ≥64 | Reserved |
HS-mode and VS-mode ECALLs use different cause values so they can be delegated separately.
When V=1, a virtual instruction exception (code 22) is normally
raised instead of an illegal instruction exception if the attempted
instruction is HS-qualified
but is prevented from executing when V=1 either due to
insufficient privilege or because the instruction is expressly disabled
by a supervisor or hypervisor CSR such as scounteren or hcounteren.
An instruction is HS-qualified if it would be valid to execute
in HS-mode (for some values of the instruction’s register operands),
assuming fields TSR and TVM of CSR mstatus are both zero.
Special rules apply for CSR instructions that access 32-bit
high-half CSRs such as cycleh and htimedeltah.
When V=1 and XLEN>32, an attempt to access a high-half
supervisor-level CSR, high-half hypervisor CSR, high-half VS CSR,
or high-half unprivileged CSR always raises an illegal instruction
exception.
And in VS-mode, if the XLEN for VU-mode is greater than 32, an attempt
to access a high-half user-level CSR (distinct from an unprivileged
CSR) always raises an illegal instruction exception.
On the other hand, when V=1 and XLEN=32, an invalid attempt to access a
high-half S-level, hypervisor, VS, or unprivileged CSR raises a virtual
instruction exception instead of an illegal instruction exception
if the same CSR instruction for the partner low-half CSR
(e.g.cycle or htimedelta) is HS-qualified.
Likewise, in VS-mode, if the XLEN for VU-mode is 32, an invalid attempt
to access a high-half user-level CSR raises a virtual instruction
exception instead of an illegal instruction exception if the same CSR
instruction for the partner low-half CSR is HS-qualified.
The RISC-V Privileged Architecture currently defines no user-level CSRs, but they might be added by a future version of this standard or by an extension.
Specifically, a virtual instruction exception is raised for the following cases:
in VS-mode, attempts to access a non-high-half counter CSR when the corresponding bit in
hcounterenis 0 and the same bit inmcounterenis 1;in VS-mode, if XLEN=32, attempts to access a high-half counter CSR when the corresponding bit in
hcounterenis 0 and the same bit inmcounterenis 1;in VU-mode, attempts to access a non-high-half counter CSR when the corresponding bit in either
hcounterenorscounterenis 0 and the same bit inmcounterenis 1;in VU-mode, if XLEN=32, attempts to access a high-half counter CSR when the corresponding bit in either
hcounterenorscounterenis 0 and the same bit inmcounterenis 1;in VS-mode or VU-mode, attempts to execute a hypervisor instruction (HLV, HLVX, HSV, or HFENCE);
in VS-mode or VU-mode, attempts to access an implemented non-high-half hypervisor CSR or VS CSR when the same access (read/write) would be allowed in HS-mode, assuming
mstatus.TVM=0;in VS-mode or VU-mode, if XLEN=32, attempts to access an implemented high-half hypervisor CSR or high-half VS CSR when the same access (read/write) to the CSR’s low-half partner would be allowed in HS-mode, assuming
mstatus.TVM=0;in VU-mode, attempts to execute WFI when
mstatus.TW=0, or to execute a supervisor instruction (SRET or SFENCE);in VU-mode, attempts to access an implemented non-high-half supervisor CSR when the same access (read/write) would be allowed in HS-mode, assuming
mstatus.TVM=0;in VU-mode, if XLEN=32, attempts to access an implemented high-half supervisor CSR when the same access to the CSR’s low-half partner would be allowed in HS-mode, assuming
mstatus.TVM=0;in VS-mode, attempts to execute WFI when
hstatus.VTW=1 andmstatus.TW=0, unless the instruction completes within an implementation-specific, bounded time;in VS-mode, attempts to execute SRET when
hstatus.VTSR=1; andin VS-mode, attempts to execute an SFENCE.VMA or SINVAL.VMA instruction or to access
satp, whenhstatus.VTVM=1.
Other extensions to the RISC-V Privileged Architecture may add to the set of circumstances that cause a virtual instruction exception when V=1.
On a virtual instruction trap, mtval or stval is written the
same as for an illegal instruction trap.
It is not unusual that hypervisors must emulate the instructions that raise virtual instruction exceptions, to support nested hypervisors or for other reasons. Machine level is expected ordinarily to delegate virtual instruction traps directly to HS-level, whereas illegal instruction traps are likely to be processed first in M-mode before being conditionally delegated (by software) to HS-level. Consequently, virtual instruction traps are expected typically to be handled faster than illegal instruction traps.
When not emulating the trapping instruction, a hypervisor should convert a virtual instruction trap into an illegal instruction exception for the guest virtual machine.
Because TSR and TVM in mstatus are intended to impact only S-mode
(HS-mode), they are ignored for determining exceptions in VS-mode.
| Priority | Exc.Code | Description |
|---|---|---|
| Highest | 3 | Instruction address breakpoint |
| During instruction address translation: | ||
| 12, 20, 1 | First encountered page fault, guest-page fault, or access fault | |
| With physical address for instruction: | ||
| 1 | Instruction access fault | |
| 2 | Illegal instruction | |
| 22 | Virtual instruction | |
| 0 | Instruction address misaligned | |
| 8, 9, 10, 11 | Environment call | |
| 3 | Environment break | |
| 3 | Load/store/AMO address breakpoint | |
| Optionally: | ||
| 4, 6 | Load/store/AMO address misaligned | |
| During address translation for an explicit memory access: | ||
| 13, 15, 21, 23, 5, 7 | First encountered page fault, guest-page fault, or access fault | |
| With physical address for an explicit memory access: | ||
| 5, 7 | Load/store/AMO access fault | |
| If not higher priority: | ||
| Lowest | 4, 6 | Load/store/AMO address misaligned |
If an instruction may raise multiple synchronous exceptions, the
decreasing priority order of Table [tab:HSyncExcPrio] indicates
which exception is taken and reported in mcause or scause.
5.6.2 Trap Entry
When a trap occurs in HS-mode or U-mode, it goes to M-mode, unless
delegated by medeleg or mideleg, in which case it goes to HS-mode.
When a trap occurs in VS-mode or VU-mode, it goes to M-mode, unless
delegated by medeleg or mideleg, in which case it goes to HS-mode,
unless further delegated by hedeleg or hideleg, in which case it
goes to VS-mode.
When a trap is taken into M-mode, virtualization mode V gets set to 0,
and fields MPV and MPP in mstatus
(or mstatush) are set according to
Table [h-mpp].
A trap into M-mode also writes fields GVA, MPIE, and MIE in
mstatus/mstatush and writes CSRs mepc, mcause,
mtval, mtval2, and mtinst.
| Previous Mode | MPV | MPP |
|---|---|---|
| U-mode | 0 | 0 |
| HS-mode | 0 | 1 |
| M-mode | 0 | 3 |
| VU-mode | 1 | 0 |
| VS-mode | 1 | 1 |
When a trap is taken into HS-mode, virtualization mode V is set to 0,
and hstatus.SPV and sstatus.SPP are
set according to Table [h-spp].
If V was 1 before the trap, field SPVP in hstatus is set the same as
sstatus.SPP;
otherwise, SPVP is left unchanged.
A trap into HS-mode also writes field GVA in hstatus, fields
SPIE and SIE in sstatus, and CSRs sepc, scause,
stval, htval, and htinst.
| Previous Mode | SPV | SPP |
|---|---|---|
| U-mode | 0 | 0 |
| HS-mode | 0 | 1 |
| VU-mode | 1 | 0 |
| VS-mode | 1 | 1 |
When a trap is taken into VS-mode, vsstatus.SPP is set according to
Table [h-vspp].
Register hstatus and the HS-level sstatus are not modified,
and the virtualization mode V remains 1.
A trap into VS-mode also writes fields SPIE and SIE in
vsstatus and writes CSRs vsepc, vscause, and
vstval.
| Previous Mode | SPP |
|---|---|
| VU-mode | 0 |
| VS-mode | 1 |
5.6.3 Transformed Instruction or Pseudoinstruction for mtinst or htinst
On any trap into M-mode or HS-mode, one of these values is written
automatically into the appropriate trap instruction CSR, mtinst or
htinst:
zero;
a transformation of the trapping instruction;
a custom value (allowed only if the trapping instruction is nonstandard); or
a special pseudoinstruction.
Except when a pseudoinstruction value is required (described later), the
value written to mtinst or htinst may always be zero,
indicating that the hardware is providing no information in the register
for this particular trap.
The value written to the trap instruction CSR serves two purposes. The first is to improve the speed of instruction emulation in a trap handler, partly by allowing the handler to skip loading the trapping instruction from memory, and partly by obviating some of the work of decoding and executing the instruction. The second purpose is to supply, via pseudoinstructions, additional information about guest-page-fault exceptions caused by implicit memory accesses done for VS-stage address translation.
A transformation of the trapping instruction is written instead of simply a copy of the original instruction in order to minimize the burden for hardware yet still provide to a trap handler the information needed to emulate the instruction. An implementation may at any time reduce its effort by substituting zero in place of the transformed instruction.
On an interrupt, the value written to the trap instruction register is always zero. On a synchronous exception, if a nonzero value is written, one of the following shall be true about the value:
Bit 0 is
1, and replacing bit 1 with1makes the value into a valid encoding of a standard instruction.In this case, the instruction that trapped is the same kind as indicated by the register value, and the register value is the transformation of the trapping instruction, as defined later. For example, if bits 1:0 are binary
11and the register value is the encoding of a standard LW (load word) instruction, then the trapping instruction is LW, and the register value is the transformation of the trapping LW instruction.Bit 0 is
1, and replacing bit 1 with1makes the value into an instruction encoding that is explicitly designated for a custom instruction (not an unused reserved encoding).This is a custom value. The instruction that trapped is a nonstandard instruction. The interpretation of a custom value is not otherwise specified by this standard.
The value is one of the special pseudoinstructions defined later, all of which have bits 1:0 equal to
00.
These three cases exclude a large number of other possible values, such
as all those having bits 1:0 equal to binary 10.
A future standard or extension may define additional cases, thus allowing
values that are currently excluded.
Software may safely treat an unrecognized value in a trap instruction
register the same as zero.
To be forward-compatible with future revisions of this standard, software
that interprets a nonzero value from mtinst or htinst must
fully verify that the value conforms to one of the cases listed above.
For instance, for RV64, discovering that bits 6:0 of mtinst are
0000011 and bits 14:12 are 010 is not sufficient to establish
that the first case applies and the trapping instruction is a standard LW
instruction;
rather, software must also confirm that bits 63:32 of mtinst are
all zeros.
A future standard might define new values for 64-bit mtinst that
are nonzero in bits 63:32 yet may coincidentally have in bits 31:0 the
same bit patterns as standard RV64 instructions.
Unlike for standard instructions, there is no requirement that the instruction encoding of a custom value be of the same “kind” as the instruction that trapped (or even have any correlation with the trapping instruction).
Table [tab:tinst-values] shows the values that may be automatically written to the trap instruction register for each standard exception cause. For exceptions that prevent the fetching of an instruction, only zero or a pseudoinstruction value may be written. A custom value may be automatically written only if the instruction that traps is nonstandard. A future standard or extension may permit other values to be written, chosen from the set of allowed values established earlier.
| Transformed | Pseudo- | |||
| Standard | Custom | instruction | ||
| Exception | Zero | Instruction | Value | Value |
| Instruction address misaligned | Yes | No | Yes | No |
| Instruction access fault | Yes | No | No | No |
| Illegal instruction | Yes | No | No | No |
| Breakpoint | Yes | No | Yes | No |
| Virtual instruction | Yes | No | Yes | No |
| Load address misaligned | Yes | Yes | Yes | No |
| Load access fault | Yes | Yes | Yes | No |
| Store/AMO address misaligned | Yes | Yes | Yes | No |
| Store/AMO access fault | Yes | Yes | Yes | No |
| Environment call | Yes | No | Yes | No |
| Instruction page fault | Yes | No | No | No |
| Load page fault | Yes | Yes | Yes | No |
| Store/AMO page fault | Yes | Yes | Yes | No |
| Instruction guest-page fault | Yes | No | No | Yes |
| Load guest-page fault | Yes | Yes | Yes | Yes |
| Store/AMO guest-page fault | Yes | Yes | Yes | Yes |
As enumerated in the table, a synchronous exception may write to the trap instruction register a standard transformation of the trapping instruction only for exceptions that arise from explicit memory accesses (from loads, stores, and AMO instructions). Accordingly, standard transformations are currently defined only for these memory-access instructions. If a synchronous trap occurs for a standard instruction for which no transformation has been defined, the trap instruction register shall be written with zero (or, under certain circumstances, with a special pseudoinstruction value).
For a standard load instruction that is not a compressed instruction and is one of LB, LBU, LH, LHU, LW, LWU, LD, FLW, FLD, FLQ, or FLH, the transformed instruction has the format shown in Figure 1.46.
For a standard store instruction that is not a compressed instruction and is one of SB, SH, SW, SD, FSW, FSD, FSQ, or FSH, the transformed instruction has the format shown in Figure 1.47.
For a standard atomic instruction (load-reserved, store-conditional, or AMO instruction), the transformed instruction has the format shown in Figure 1.48.
For a standard virtual-machine load/store instruction (HLV, HLVX, or HSV), the transformed instruction has the format shown in Figure 1.49.
In all the transformed instructions above, the Addr. Offset field that
replaces the instruction’s rs1 field in bits 19:15 is the positive
difference between the faulting virtual address (written to mtval
or stval) and the original virtual address.
This difference can be nonzero only for a misaligned memory access.
Note also that, for basic loads and stores, the transformations replace
the instruction’s immediate offset fields with zero.
For a standard compressed instruction (16-bit size), the transformed instruction is found as follows:
Expand the compressed instruction to its 32-bit equivalent.
Transform the 32-bit equivalent instruction.
Replace bit 1 with a
0.
Bits 1:0 of a transformed standard instruction will be binary 01 if
the trapping instruction is compressed and 11 if not.
In decoding the contents of mtinst or htinst, once software
has determined that the register contains the encoding of a standard
basic load (LB, LBU, LH, LHU, LW, LWU, LD, FLW, FLD, FLQ, or FLH) or basic
store (SB, SH, SW, SD, FSW, FSD, FSQ, or FSH), it is not necessary to confirm
also that the immediate offset fields (31:25, and 24:20 or 11:7) are
zeros.
The knowledge that the register’s value is the encoding of a basic
load/store is sufficient to prove that the trapping instruction is of the
same kind.
A future version of this standard may add information to the fields that are currently zeros. However, for backwards compatiblity, any such information will be for performance purposes only and can safely be ignored.
For guest-page faults, the trap instruction register is written with a
special pseudoinstruction value if:
(a) the fault is caused by an implicit memory access for VS-stage address
translation, and
(b) a nonzero value (the faulting guest physical address) is written to
mtval2 or htval.
If both conditions are met, the value written to mtinst or
htinst must be taken from Table [tab:pseudoinsts];
zero is not allowed.
| Value | Meaning |
|---|---|
0x00002000 |
32-bit read for VS-stage address translation (RV32) |
0x00002020 |
32-bit write for VS-stage address translation (RV32) |
0x00003000 |
64-bit read for VS-stage address translation (RV64) |
0x00003020 |
64-bit write for VS-stage address translation (RV64) |
The defined pseudoinstruction values are designed to correspond closely with the encodings of basic loads and stores, as illustrated by Table [tab:pseudoinsts-basis].
| Encoding | Instruction |
|---|---|
0x00002003 |
lw x0,0(x0) |
0x00002023 |
sw x0,0(x0) |
0x00003003 |
ld x0,0(x0) |
0x00003023 |
sd x0,0(x0) |
A write pseudoinstruction (0x00002020 or 0x00003020)
is used for the case that the machine is attempting automatically to
update bits A and/or D in VS-level page tables.
All other implicit memory accesses for VS-stage address translation will
be reads.
If a machine never automatically updates bits A or D in VS-level page
tables (leaving this to software), the write case will never
arise.
The fact that such a page table update must actually be atomic, not just
a simple write, is ignored for the pseudoinstruction.
If the conditions that necessitate a pseudoinstruction value can ever
occur for M-mode, then mtinst cannot be entirely read-only zero;
and likewise for HS-mode and htinst.
However, in that case, the trap instruction registers may minimally
support only values 0 and 0x00002000 or 0x00003000, and
possibly 0x00002020 or 0x00003020, requiring as few as one or
two flip-flops in hardware, per register.
There is no harm here in ignoring the atomicity requirement for page table updates, because a hypervisor is not expected in these circumstances to emulate an implicit memory access that fails. Rather, the hypervisor is given enough information about the faulting access to be able to make the memory accessible (e.g. by restoring a missing page of virtual memory) before resuming execution by retrying the faulting instruction.
5.6.4 Trap Return
The MRET instruction is used to return from a trap taken into M-mode.
MRET first determines what the new privilege mode will be according to
the values of MPP and MPV in mstatus or mstatush, as encoded in
Table [h-mpp].
MRET then in mstatus/mstatush sets MPV=0, MPP=0, MIE=MPIE, and MPIE=1.
Lastly, MRET sets the privilege mode as previously
determined, and sets pc=mepc.
The SRET instruction is used to return from a trap taken into HS-mode or VS-mode. Its behavior depends on the current virtualization mode.
When executed in M-mode or HS-mode (i.e., V=0), SRET first determines
what the new privilege mode will be according to the values in
hstatus.SPV and sstatus.SPP, as encoded in Table [h-spp].
SRET then sets hstatus.SPV=0, and in sstatus sets SPP=0,
SIE=SPIE, and SPIE=1.
Lastly, SRET sets the privilege mode as previously
determined, and sets pc=sepc.
When executed in VS-mode (i.e., V=1), SRET sets the privilege mode according to
Table [h-vspp], in vsstatus sets SPP=0, SIE=SPIE, and SPIE=1, and
lastly sets pc=vsepc.
The baseline privileged architecture is designed to simplify the use of classic virtualization techniques, where a guest OS is run at user-level, as the few privileged instructions can be easily detected and trapped. The hypervisor extension improves virtualization performance by reducing the frequency of these traps.
The hypervisor extension has been designed to be efficiently emulable on platforms that do not implement the extension, by running the hypervisor in S-mode and trapping into M-mode for hypervisor CSR accesses and to maintain shadow page tables. The majority of CSR accesses for type-2 hypervisors are valid S-mode accesses so need not be trapped. Hypervisors can support nested virtualization analogously.