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The RISC-V Instruction Set Manual, Volume II: Privileged Architecture

4 Supervisor-Level ISA, Version 1.12

This chapter describes the RISC-V supervisor-level architecture, which contains a common core that is used with various supervisor-level address translation and protection schemes.

Supervisor mode is deliberately restricted in terms of interactions with underlying physical hardware, such as physical memory and device interrupts, to support clean virtualization. In this spirit, certain supervisor-level facilities, including requests for timer and interprocessor interrupts, are provided by implementation-specific mechanisms. In some systems, a supervisor execution environment (SEE) provides these facilities in a manner specified by a supervisor binary interface (SBI). Other systems supply these facilities directly, through some other implementation-defined mechanism.

4.1 Supervisor CSRs

A number of CSRs are provided for the supervisor.

The supervisor should only view CSR state that should be visible to a supervisor-level operating system. In particular, there is no information about the existence (or non-existence) of higher privilege levels (machine level or other) visible in the CSRs accessible by the supervisor.

Many supervisor CSRs are a subset of the equivalent machine-mode CSR, and the machine-mode chapter should be read first to help understand the supervisor-level CSR descriptions.

4.1.1 Supervisor Status Register (sstatus)

The sstatus register is an SXLEN-bit read/write register formatted as shown in Figure 1.1 for RV32 and Figure 1.2 for RV64. The sstatus register keeps track of the processor’s current operating state.

Supervisor-mode status register (sstatus) for RV32.
Supervisor-mode status register (sstatus) for RV64.

The SPP bit indicates the privilege level at which a hart was executing before entering supervisor mode. When a trap is taken, SPP is set to 0 if the trap originated from user mode, or 1 otherwise. When an SRET instruction (see Section [otherpriv]) is executed to return from the trap handler, the privilege level is set to user mode if the SPP bit is 0, or supervisor mode if the SPP bit is 1; SPP is then set to 0.

The SIE bit enables or disables all interrupts in supervisor mode. When SIE is clear, interrupts are not taken while in supervisor mode. When the hart is running in user-mode, the value in SIE is ignored, and supervisor-level interrupts are enabled. The supervisor can disable individual interrupt sources using the sie CSR.

The SPIE bit indicates whether supervisor interrupts were enabled prior to trapping into supervisor mode. When a trap is taken into supervisor mode, SPIE is set to SIE, and SIE is set to 0. When an SRET instruction is executed, SIE is set to SPIE, then SPIE is set to 1.

The sstatus register is a subset of the mstatus register. In a straightforward implementation, reading or writing any field in sstatus is equivalent to reading or writing the homonymous field in mstatus.

4.1.1.1 Base ISA Control in sstatus Register

The UXL field controls the value of XLEN for U-mode, termed UXLEN, which may differ from the value of XLEN for S-mode, termed SXLEN. The encoding of UXL is the same as that of the MXL field of misa, shown in Table [misabase].

For RV32 systems, the UXL field does not exist, and UXLEN=32. For RV64 systems, it is a WARL field that encodes the current value of UXLEN. In particular, the implementation may hardwire UXL so that UXLEN=SXLEN.

If UXLEN  SXLEN, instructions executed in the narrower mode must ignore source register operand bits above the configured XLEN, and must sign-extend results to fill the widest supported XLEN in the destination register.

If UXLEN < SXLEN, user-mode instruction-fetch addresses and load and store effective addresses are taken modulo 2UXLEN. For example, when UXLEN=32 and SXLEN=64, user-mode memory accesses reference the lowest 4 GiB of the address space.

4.1.1.2 Memory Privilege in sstatus Register

The MXR (Make eXecutable Readable) bit modifies the privilege with which loads access virtual memory. When MXR=0, only loads from pages marked readable (R=1 in Figure 1.18) will succeed. When MXR=1, loads from pages marked either readable or executable (R=1 or X=1) will succeed. MXR has no effect when page-based virtual memory is not in effect.

The SUM (permit Supervisor User Memory access) bit modifies the privilege with which S-mode loads and stores access virtual memory. When SUM=0, S-mode memory accesses to pages that are accessible by U-mode (U=1 in Figure 1.18) will fault. When SUM=1, these accesses are permitted. SUM has no effect when page-based virtual memory is not in effect, nor when executing in U-mode. Note that S-mode can never execute instructions from user pages, regardless of the state of SUM.

The SUM mechanism prevents supervisor software from inadvertently accessing user memory. Operating systems can execute the majority of code with SUM clear; the few code segments that should access user memory can temporarily set SUM.

The SUM mechanism does not avail S-mode software of permission to execute instructions in user code pages. Legitimate uses cases for execution from user memory in supervisor context are rare in general and nonexistent in POSIX environments. However, bugs in supervisors that lead to arbitrary code execution are much easier to exploit if the supervisor exploit code can be stored in a user buffer at a virtual address chosen by an attacker.

Some non-POSIX single address space operating systems do allow certain privileged software to partially execute in supervisor mode, while most programs run in user mode, all in a shared address space. This use case can be realized by mapping the physical code pages at multiple virtual addresses with different permissions, possibly with the assistance of the instruction page-fault handler to direct supervisor software to use the alternate mapping.

4.1.1.3 Endianness Control in sstatus Register

The UBE bit is a WARL field that controls the endianness of explicit memory accesses made from U-mode, which may differ from the endianness of memory accesses in S-mode. An implementation may hardwire UBE to specify always the same endianness as for S-mode.

UBE controls whether explicit load and store memory accesses made from U-mode are little-endian (UBE=0) or big-endian (UBE=1).

UBE has no effect on instruction fetches, which are implicit memory accesses that are always little-endian.

For implicit accesses to supervisor-level memory management data structures, such as page tables, S-mode endianness always applies and UBE is ignored.

Standard RISC-V ABIs are expected to be purely little-endian-only or big-endian-only, with no accommodation for mixing endianness. Nevertheless, endianness control has been defined so as to permit an OS of one endianness to execute user-mode programs of the opposite endianness.

4.1.2 Supervisor Trap Vector Base Address Register (stvec)

The stvec register is an SXLEN-bit read/write register that holds trap vector configuration, consisting of a vector base address (BASE) and a vector mode (MODE).

Supervisor trap vector base address register (stvec).

The BASE field in stvec is a WARL field that can hold any valid virtual or physical address, subject to the following alignment constraints: the address must be 4-byte aligned, and MODE settings other than Direct might impose additional alignment constraints on the value in the BASE field.

Value Name Description
0 Direct All exceptions set pc to BASE.
1 Vectored Asynchronous interrupts set pc to BASE+4×cause.
2 Reserved

The encoding of the MODE field is shown in Table [stvec-mode]. When MODE=Direct, all traps into supervisor mode cause the pc to be set to the address in the BASE field. When MODE=Vectored, all synchronous exceptions into supervisor mode cause the pc to be set to the address in the BASE field, whereas interrupts cause the pc to be set to the address in the BASE field plus four times the interrupt cause number. For example, a supervisor-mode timer interrupt (see Table [scauses]) causes the pc to be set to BASE+0x14. Setting MODE=Vectored may impose a stricter alignment constraint on BASE.

4.1.3 Supervisor Interrupt Registers (sip and sie)

The sip register is an SXLEN-bit read/write register containing information on pending interrupts, while sie is the corresponding SXLEN-bit read/write register containing interrupt enable bits. Interrupt cause number i (as reported in CSR scause, Section 1.1.8) corresponds with bit i in both sip and sie. Bits 15:0 are allocated to standard interrupt causes only, while bits 16 and above are available for platform or custom use.

Supervisor interrupt-pending register (sip).
Supervisor interrupt-enable register (sie).

An interrupt i will be taken if bit i is set in both sip and sie, and if supervisor-level interrupts are globally enabled. Supervisor-level interrupts are globally enabled if the hart’s current privilege mode is less than S, or if the current privilege mode is S and the SIE bit in the sstatus register is set.

Each individual bit in register sip may be writable or may be read-only. When bit i in sip is writable, a pending interrupt i can be cleared by writing 0 to this bit. If interrupt i can become pending but bit i in sip is read-only, the implementation must provide some other mechanism for clearing the pending interrupt (which may involve a call to the execution environment).

A bit in sie must be writable if the corresponding interrupt can ever become pending. Bits of sie that are not writable must be hardwired to zero.

The standard portions (bits 15:0) of registers sip and sie are formatted as shown in Figures 1.6 and 1.7 respectively.

Standard portion (bits 15:0) of sip.
Standard portion (bits 15:0) of sie.

Bits sip.SEIP and sie.SEIE are the interrupt-pending and interrupt-enable bits for supervisor-level external interrupts. If implemented, SEIP is read-only in sip, and is set and cleared by the execution environment, typically through a platform-specific interrupt controller.

Bits sip.STIP and sie.STIE are the interrupt-pending and interrupt-enable bits for supervisor-level timer interrupts. If implemented, STIP is read-only in sip, and is set and cleared by the execution environment.

Bits sip.SSIP and sie.SSIE are the interrupt-pending and interrupt-enable bits for supervisor-level software interrupts. If implemented, SSIP is writable in sip. A supervisor-level software interrupt is triggered on the current hart by writing 1 to SSIP, while a pending supervisor-level software interrupt can be cleared by writing 0 to SSIP.

Interprocessor interrupts are sent to other harts by implementation-specific means, which will ultimately cause the SSIP bit to be set in the recipient hart’s sip register.

Each standard interrupt type (SEI, STI, or SSI) may not be implemented, in which case the corresponding interrupt-pending and interrupt-enable bits are hardwired to zeros. All bits in sip and sie are WARL fields. The implemented interrupts may be found by writing one to every bit location in sie, then reading back to see which bit positions hold a one.

The sip and sie registers are subsets of the mip and mie registers. Reading any implemented field, or writing any writable field, of sip/sie effects a read or write of the homonymous field of mip/mie.

Bits 3, 7, and 11 of sip and sie correspond to the machine-mode software, timer, and external interrupts, respectively. Since most platforms will choose not to make these interrupts delegatable from M-mode to S-mode, they are shown as hardwired to 0 in Figures 1.6 and 1.7.

Multiple simultaneous interrupts destined for supervisor mode are handled in the following decreasing priority order: SEI, SSI, STI. Synchronous exceptions are of lower priority than all interrupts.

4.1.4 Supervisor Timers and Performance Counters

Supervisor software uses the same hardware performance monitoring facility as user-mode software, including the time, cycle, and instret CSRs. The implementation should provide a mechanism to modify the counter values.

The implementation must provide a facility for scheduling timer interrupts in terms of the real-time counter, time.

4.1.5 Counter-Enable Register (scounteren)

Counter-enable register (scounteren).

The counter-enable register scounteren is a 32-bit register that controls the availability of the hardware performance monitoring counters to U-mode.

When the CY, TM, IR, or HPMn bit in the scounteren register is clear, attempts to read the cycle, time, instret, or hpmcountern register while executing in U-mode will cause an illegal instruction exception. When one of these bits is set, access to the corresponding register is permitted.

scounteren must be implemented. However, any of the bits may contain a hardwired value of zero, indicating reads to the corresponding counter will cause an exception when executing in U-mode. Hence, they are effectively WARL fields.

4.1.6 Supervisor Scratch Register (sscratch)

The sscratch register is an SXLEN-bit read/write register, dedicated for use by the supervisor. Typically, sscratch is used to hold a pointer to the hart-local supervisor context while the hart is executing user code. At the beginning of a trap handler, sscratch is swapped with a user register to provide an initial working register.

Supervisor Scratch Register.

4.1.7 Supervisor Exception Program Counter (sepc)

sepc is an SXLEN-bit read/write register formatted as shown in Figure 1.10. The low bit of sepc (sepc[0]) is always zero. On implementations that support only IALIGN=32, the two low bits (sepc[1:0]) are always zero.

If an implementation allows IALIGN to be either 16 or 32 (by changing CSR misa, for example), then, whenever IALIGN=32, bit sepc[1] is masked on reads so that it appears to be 0. This masking occurs also for the implicit read by the SRET instruction. Though masked, sepc[1] remains writable when IALIGN=32.

sepc is a WARL register that must be able to hold all valid virtual addresses. It need not be capable of holding all possible invalid addresses. Implementations may convert some invalid address patterns into other invalid addresses prior to writing them to sepc.

When a trap is taken into S-mode, sepc is written with the virtual address of the instruction that was interrupted or that encountered the exception. Otherwise, sepc is never written by the implementation, though it may be explicitly written by software.

Supervisor exception program counter register.

4.1.8 Supervisor Cause Register (scause)

The scause register is an SXLEN-bit read-write register formatted as shown in Figure 1.11. When a trap is taken into S-mode, scause is written with a code indicating the event that caused the trap. Otherwise, scause is never written by the implementation, though it may be explicitly written by software.

The Interrupt bit in the scause register is set if the trap was caused by an interrupt. The Exception Code field contains a code identifying the last exception. Table [scauses] lists the possible exception codes for the current supervisor ISAs. The Exception Code is a WLRL field. It is required to hold the values 0–31 (i.e., bits 4–0 must be implemented), but otherwise it is only guaranteed to hold supported exception codes.

Supervisor Cause register scause.
Interrupt Exception Code Description
1 0 Reserved
1 1 Supervisor software interrupt
1 2–4 Reserved
1 5 Supervisor timer interrupt
1 6–8 Reserved
1 9 Supervisor external interrupt
1 10–15 Reserved
1 16 Available for platform 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
0 9 Environment call from S-mode
0 10–11 Reserved
0 12 Instruction page fault
0 13 Load page fault
0 14 Reserved
0 15 Store/AMO page fault
0 16–23 Reserved
0 24–31 Available for custom use
0 32–47 Reserved
0 48–63 Available for custom use
0 64 Reserved

4.1.9 Supervisor Trap Value (stval) Register

The stval register is an SXLEN-bit read-write register formatted as shown in Figure 1.12. When a trap is taken into S-mode, stval is written with exception-specific information to assist software in handling the trap. Otherwise, stval is never written by the implementation, though it may be explicitly written by software. The hardware platform will specify which exceptions must set stval informatively and which may unconditionally set it to zero.

When a hardware breakpoint is triggered, or an instruction, load, or store address-misaligned, access-fault, or page-fault exception occurs, stval is written with the faulting virtual address. On an illegal instruction trap, stval may be written with the first XLEN or ILEN bits of the faulting instruction as described below. For other exceptions, stval is set to zero, but a future standard may redefine stval’s setting for other exceptions.

Supervisor Trap Value register.

For misaligned loads and stores that cause access-fault or page-fault exceptions, stval will contain the virtual address of the portion of the access that caused the fault. For instruction access-fault or page-fault exceptions on systems with variable-length instructions, stval will contain the virtual address of the portion of the instruction that caused the fault while sepc will point to the beginning of the instruction.

The stval register can optionally also be used to return the faulting instruction bits on an illegal instruction exception ( sepc points to the faulting instruction in memory).

If this feature is not provided, then stval is set to zero on an illegal instruction fault.

If this feature is provided, after an illegal instruction trap, stval will contain the shortest of:

the actual faulting instruction

the first ILEN bits of the faulting instruction

the first XLEN bits of the faulting instruction

The value loaded into stval is right-justified and all unused upper bits are cleared to zero.

stval is a WARL register that must be able to hold all valid virtual addresses and the value 0. It need not be capable of holding all possible invalid addresses. Implementations may convert some invalid address patterns into other invalid addresses prior to writing them to stval. If the feature to return the faulting instruction bits is implemented, stval must also be able to hold all values less than 2N, where N is the smaller of XLEN and ILEN.

4.1.10 Supervisor Address Translation and Protection (satp) Register

The satp register is an SXLEN-bit read/write register, formatted as shown in Figure 1.13 for SXLEN=32 and Figure 1.14 for SXLEN=64, which controls supervisor-mode address translation and protection. This register holds the physical page number (PPN) of the root page table, i.e., its supervisor physical address divided by 4 KiB; an address space identifier (ASID), which facilitates address-translation fences on a per-address-space basis; and the MODE field, which selects the current address-translation scheme. Further details on the access to this register are described in Section [virt-control].

RV32 Supervisor address translation and protection register satp.

Storing a PPN in satp, rather than a physical address, supports a physical address space larger than 4 GiB for RV32.

The satp.PPN field might not be capable of holding all physical page numbers. Some platform standards might place constraints on the values satp.PPN may assume, e.g., by requiring that all physical page numbers corresponding to main memory be representable.

RV64 Supervisor address translation and protection register satp, for MODE values Bare, Sv39, and Sv48.

We store the ASID and the page table base address in the same CSR to allow the pair to be changed atomically on a context switch. Swapping them non-atomically could pollute the old virtual address space with new translations, or vice-versa. This approach also slightly reduces the cost of a context switch.

Table 1.15 shows the encodings of the MODE field for RV32 and RV64. When MODE=Bare, supervisor virtual addresses are equal to supervisor physical addresses, and there is no additional memory protection beyond the physical memory protection scheme described in Section [sec:pmp]. When MODE=Bare, patterns other than zero in the remaining fields of satp (bits 30–0 for RV32, or bits 59–0 for RV64) are reserved for future use. To select MODE=Bare, software must write zero to these fields.

Version 1.11 of this standard stated that the remaining fields in satp had no effect when MODE=Bare. Making these fields reserved facilitates future definition of additional translation and protection modes, particularly in RV32, for which all patterns of the existing MODE field have already been allocated.

For RV32, the only other valid setting for MODE is Sv32, a paged virtual-memory scheme described in Section 1.3.

For RV64, two paged virtual-memory schemes are defined: Sv39 and Sv48, described in Sections 1.4 and 1.5, respectively. Two additional schemes, Sv57 and Sv64, will be defined in a later version of this specification. The remaining MODE settings are reserved for future use and may define different interpretations of the other fields in satp.

Implementations are not required to support all MODE settings, and if satp is written with an unsupported MODE, the entire write has no effect; no fields in satp are modified.

Encoding of satp MODE field.

The number of ASID bits is  and may be zero. The number of implemented ASID bits, termed ASIDLEN , may be determined by writing one to every bit position in the ASID field, then reading back the value in satp to see which bit positions in the ASID field hold a one. The least-significant bits of ASID are implemented first: that is, if ASIDLEN > 0, ASID[ASIDLEN-1:0] is writable. The maximal value of ASIDLEN, termed ASIDMAX, is 9 for Sv32 or 16 for Sv39 and Sv48.

For many applications, the choice of page size has a substantial performance impact. A large page size increases TLB reach and loosens the associativity constraints on virtually-indexed, physically-tagged caches. At the same time, large pages exacerbate internal fragmentation, wasting physical memory and possibly cache capacity.

After much deliberation, we have settled on a conventional page size of 4 KiB for both RV32 and RV64. We expect this decision to ease the porting of low-level runtime software and device drivers. The TLB reach problem is ameliorated by transparent superpage support in modern operating systems [transparent-superpages]). Additionally, multi-level TLB hierarchies are quite inexpensive relative to the multi-level cache hierarchies whose address space they map.

Note that writing satp does not imply any ordering constraints between page-table updates and subsequent address translations. If the new address space’s page tables have been modified, or if an ASID is reused, it may be necessary to execute an SFENCE.VMA instruction (see Section 1.2.1) after writing satp.

Not imposing upon implementations to flush address-translation caches upon satp writes reduces the cost of context switches, provided a sufficiently large ASID space.

4.2 Supervisor Instructions

In addition to the SRET instruction defined in Section [otherpriv], one new supervisor-level instruction is provided.

4.2.1 Supervisor Memory-Management Fence Instruction

image

The supervisor memory-management fence instruction SFENCE.VMA is used to synchronize updates to in-memory memory-management data structures with current execution. Instruction execution causes implicit reads and writes to these data structures; however, these implicit references are ordinarily not ordered with respect to explicit loads and stores. Executing an SFENCE.VMA instruction guarantees that any previous stores already visible to the current RISC-V hart are ordered before all subsequent implicit references from that hart to the memory-management data structures. Further details on the behavior of this instruction are described in Section [virt-control] and Section [pmp-vmem].

The SFENCE.VMA is used to flush any local hardware caches related to address translation. It is specified as a fence rather than a TLB flush to provide cleaner semantics with respect to which instructions are affected by the flush operation and to support a wider variety of dynamic caching structures and memory-management schemes. SFENCE.VMA is also used by higher privilege levels to synchronize page table writes and the address translation hardware.

SFENCE.VMA orders only the local hart’s implicit references to the memory-management data structures.

Consequently, other harts must be notified separately when the memory-management data structures have been modified. One approach is to use 1) a local data fence to ensure local writes are visible globally, then 2) an interprocessor interrupt to the other thread, then 3) a local SFENCE.VMA in the interrupt handler of the remote thread, and finally 4) signal back to originating thread that operation is complete. This is, of course, the RISC-V analog to a TLB shootdown.

For the common case that the translation data structures have only been modified for a single address mapping (i.e., one page or superpage), rs1 can specify a virtual address within that mapping to effect a translation fence for that mapping only. Furthermore, for the common case that the translation data structures have only been modified for a single address-space identifier, rs2 can specify the address space. The behavior of SFENCE.VMA depends on rs1 and rs2 as follows:

  • If rs1=x0 and rs2=x0, the fence orders all reads and writes made to any level of the page tables, for all address spaces.

  • If rs1=x0 and rs2x0, the fence orders all reads and writes made to any level of the page tables, but only for the address space identified by integer register rs2. Accesses to global mappings (see Section 1.3.1) are not ordered.

  • If rs1x0 and rs2=x0, the fence orders only reads and writes made to the leaf page table entry corresponding to the virtual address in rs1, for all address spaces.

  • If rs1x0 and rs2x0, the fence orders only reads and writes made to the leaf page table entry corresponding to the virtual address in rs1, for the address space identified by integer register rs2. Accesses to global mappings are not ordered.

When rs2x0, bits SXLEN-1:ASIDMAX of the value held in rs2 are reserved for future use and 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 can ignore the virtual address in rs1 and the ASID value in rs2 and always perform a global fence.

Implementations may perform implicit reads of the translation data structures pointed to by the current satp register arbitrarily early and speculatively. The results of these reads may be held in an incoherent cache but not shared with other harts. Cache entries may only be established for the ASID currently loaded into the satp register, or for global entries. The cache may only satisfy implicit reads for entries that have been established for the ASID currently loaded into satp, or for global entries. Changes in the satp register do not necessarily flush any such translation caches. To ensure the implicit reads observe writes to the same memory locations, an SFENCE.VMA instruction must be executed after the writes to flush the relevant cached translations.

Implementations must only perform implicit reads of the translation data structures pointed to by the current contents of the satp register, and must only raise exceptions for implicit accesses that are generated as a result of instruction execution, not those that are performed speculatively.

Changes to the sstatus fields SUM and MXR take effect immediately, without the need to execute an SFENCE.VMA instruction.

The following common situations typically require executing an SFENCE.VMA instruction:

  • When software recycles an ASID (i.e., reassociates it with a different page table), it should first change satp to point to the new page table using the recycled ASID, then execute SFENCE.VMA with rs1=x0 and rs2 set to the recycled ASID. Alternatively, software can execute the same SFENCE.VMA instruction while a different ASID is loaded into satp, provided the next time satp is loaded with the recycled ASID, it is simultaneously loaded with the new page table.

  • If the implementation does not provide ASIDs, or software chooses to always use ASID 0, then after every satp write, software should execute SFENCE.VMA with rs1=x0. In the common case that no global translations have been modified, rs2 should be set to a register other than x0 but which contains the value zero, so that global translations are not flushed.

  • If software modifies a non-leaf PTE, it should execute SFENCE.VMA with rs1=x0. If any PTE along the traversal path had its G bit set, rs2 must be x0; otherwise, rs2 should be set to the ASID for which the translation is being modified.

  • If software modifies a leaf PTE, it should execute SFENCE.VMA with rs1 set to a virtual address within the page. If any PTE along the traversal path had its G bit set, rs2 must be x0; otherwise, rs2 should be set to the ASID for which the translation is being modified.

  • For the special cases of increasing the permissions on a leaf PTE and changing an invalid PTE to a valid leaf, software may choose to execute the SFENCE.VMA lazily. After modifying the PTE but before executing SFENCE.VMA, either the new or old permissions will be used. In the latter case, a page-fault exception might occur, at which point software should execute SFENCE.VMA in accordance with the previous bullet point.

For forward compatibility with future versions of this specification, supervisor software should adhere to the following constraints on ASID usage. Supervisor software that uses ASIDs should use a nonzero ASID value to refer to the same address space across all harts in the SEE and should not use an ASID value of 0. Supervisor software that does not use ASIDs should always set the ASID field in the satp CSR to 0.

A future extension may define ASIDs to be global across the SEE, facilitating such optimizations as multicast TLB shootdown and sharing translation caches between harts.

The extension will provide a mechanism so that existing supervisor software that always uses ASID 0 will continue to function as it currently does.

4.3 Sv32: Page-Based 32-bit Virtual-Memory Systems

When Sv32 is written to the MODE field in the satp register (see Section 1.1.10), the supervisor operates in a 32-bit paged virtual-memory system. In this mode, supervisor and user virtual addresses are translated into supervisor physical addresses by traversing a radix-tree page table. Sv32 is supported on RV32 systems and is designed to include mechanisms sufficient for supporting modern Unix-based operating systems.

The initial RISC-V paged virtual-memory architectures have been designed as straightforward implementations to support existing operating systems. We have architected page table layouts to support a hardware page-table walker. Software TLB refills are a performance bottleneck on high-performance systems, and are especially troublesome with decoupled specialized coprocessors. An implementation can choose to implement software TLB refills using a machine-mode trap handler as an extension to M-mode.

4.3.1 Addressing and Memory Protection

Sv32 implementations support a 32-bit virtual address space, divided into 4 KiB pages. An Sv32 virtual address is partitioned into a virtual page number (VPN) and page offset, as shown in Figure 1.16. When Sv32 virtual memory mode is selected in the MODE field of the satp register, supervisor virtual addresses are translated into supervisor physical addresses via a two-level page table. The 20-bit VPN is translated into a 22-bit physical page number (PPN), while the 12-bit page offset is untranslated. The resulting supervisor-level physical addresses are then checked using any physical memory protection structures (Sections [sec:pmp]), before being directly converted to machine-level physical addresses.

Sv32 virtual address.
Sv32 physical address.
Sv32 page table entry.

Sv32 page tables consist of 210 page-table entries (PTEs), each of four bytes. A page table is exactly the size of a page and must always be aligned to a page boundary. The physical page number of the root page table is stored in the satp register.

The PTE format for Sv32 is shown in Figures 1.18. The V bit indicates whether the PTE is valid; if it is 0, all other bits in the PTE are don’t-cares and may be used freely by software. The permission bits, R, W, and X, indicate whether the page is readable, writable, and executable, respectively. When all three are zero, the PTE is a pointer to the next level of the page table; otherwise, it is a leaf PTE. Writable pages must also be marked readable; the contrary combinations are reserved for future use. Table [pteperm] summarizes the encoding of the permission bits.

X W R Meaning
0 0 0 Pointer to next level of page table.
0 0 1 Read-only page.
0 1 0 Reserved for future use.
0 1 1 Read-write page.
1 0 0 Execute-only page.
1 0 1 Read-execute page.
1 1 0 Reserved for future use.
1 1 1 Read-write-execute page.

Attempting to fetch an instruction from a page that does not have execute permissions raises a fetch page-fault exception. Attempting to execute a load or load-reserved instruction whose effective address lies within a page without read permissions raises a load page-fault exception. Attempting to execute a store, store-conditional (regardless of success), or AMO instruction whose effective address lies within a page without write permissions raises a store page-fault exception.

AMOs never raise load page-fault exceptions. Since any unreadable page is also unwritable, attempting to perform an AMO on an unreadable page always raises a store page-fault exception.

The U bit indicates whether the page is accessible to user mode. U-mode software may only access the page when U=1. If the SUM bit in the sstatus register is set, supervisor mode software may also access pages with U=1. However, supervisor code normally operates with the SUM bit clear, in which case, supervisor code will fault on accesses to user-mode pages. Irrespective of SUM, the supervisor may not execute code on pages with U=1.

An alternative PTE format would support different permissions for supervisor and user. We omitted this feature because it would be largely redundant with the SUM mechanism (see Section 1.1.1.2) and would require more encoding space in the PTE.

The G bit designates a global mapping. Global mappings are those that exist in all address spaces. For non-leaf PTEs, the global setting implies that all mappings in the subsequent levels of the page table are global. Note that failing to mark a global mapping as global merely reduces performance, whereas marking a non-global mapping as global is a software bug that, after switching to an address space with a different non-global mapping for that address range, can unpredictably result in either mapping being used.

Global mappings need not be stored redundantly in address-translation caches for multiple ASIDs. Additionally, they need not be flushed from local address-translation caches when an SFENCE.VMA instruction is executed with rs2x0.

The RSW field is reserved for use by supervisor software; the implementation shall ignore this field.

Each leaf PTE contains an accessed (A) and dirty (D) bit. The A bit indicates the virtual page has been read, written, or fetched from since the last time the A bit was cleared. The D bit indicates the virtual page has been written since the last time the D bit was cleared.

Two schemes to manage the A and D bits are permitted:

  • When a virtual page is accessed and the A bit is clear, or is written and the D bit is clear, a page-fault exception is raised.

  • When a virtual page is accessed and the A bit is clear, or is written and the D bit is clear, the implementation sets the corresponding bit(s) in the PTE. The PTE update must be atomic with respect to other accesses to the PTE, and must atomically check that the PTE is valid and grants sufficient permissions. The PTE update must be exact (i.e., not speculative), and observed in program order by the local hart. Furthermore, the PTE update must appear in the global memory order no later than the explicit memory access, or any subsequent explicit memory access to that virtual page by the local hart. The ordering on loads and stores provided by FENCE instructions and the acquire/release bits on atomic instructions also orders the PTE updates associated with those loads and stores as observed by remote harts.

    The PTE update is not required to be atomic with respect to the explicit memory access that caused the update, and the sequence is interruptible. However, the hart must not perform the explicit memory access before the PTE update is globally visible.

All harts in a system must employ the same PTE-update scheme as each other.

Mandating that the PTE updates to be exact, atomic, and in program order simplifies the specification, and makes the feature more useful for system software. Simple implementations may instead generate page-fault exceptions.

The A and D bits are never cleared by the implementation. If the supervisor software does not rely on accessed and/or dirty bits, e.g. if it does not swap memory pages to secondary storage or if the pages are being used to map I/O space, it should always set them to 1 in the PTE to improve performance.

Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv32 supports 4 MiB megapages. A megapage must be virtually and physically aligned to a 4 MiB boundary; a page-fault exception is raised if the physical address is insufficiently aligned.

For non-leaf PTEs, the D, A, and U bits are reserved for future standard use and must be cleared by software for forward compatibility.

For implementations with both page-based virtual memory and the “A” standard extension, the LR/SC reservation set must lie completely within a single base page (i.e., a naturally aligned 4 KiB region).

4.3.2 Virtual Address Translation Process

A virtual address va is translated into a physical address pa as follows:

  1. Let a be ${\tt satp}.ppn \times \textrm{PAGESIZE}$, and let i = LEVELS − 1. (For Sv32, PAGESIZE=212 and LEVELS=2.)

  2. Let pte be the value of the PTE at address a + va.vpn[i] × PTESIZE. (For Sv32, PTESIZE=4.) If accessing pte violates a PMA or PMP check, raise an access-fault exception corresponding to the original access type.

  3. If pte.v = 0, or if pte.r = 0 and pte.w = 1, stop and raise a page-fault exception corresponding to the original access type.

  4. Otherwise, the PTE is valid. If pte.r = 1 or pte.x = 1, go to step 5. Otherwise, this PTE is a pointer to the next level of the page table. Let i = i − 1. If i < 0, stop and raise a page-fault exception corresponding to the original access type. Otherwise, let a = pte.ppn × PAGESIZE and go to step 2.

  5. A leaf PTE has been found. Determine if the requested memory access is allowed by the pte.r, pte.w, pte.x, and pte.u bits, given the current privilege mode and the value of the SUM and MXR fields of the mstatus register. If not, stop and raise a page-fault exception corresponding to the original access type.

  6. If i > 0 and pte.ppn[i − 1 : 0] ≠ 0, this is a misaligned superpage; stop and raise a page-fault exception corresponding to the original access type.

  7. If pte.a = 0, or if the memory access is a store and pte.d = 0, either raise a page-fault exception corresponding to the original access type, or:

    • Set pte.a to 1 and, if the memory access is a store, also set pte.d to 1.

    • If this access violates a PMA or PMP check, raise an access-fault exception corresponding to the original access type.

    • This update and the loading of pte in step 2 must be atomic; in particular, no intervening store to the PTE may be perceived to have occurred in-between.

  8. The translation is successful. The translated physical address is given as follows:

    • pa.pgoff = va.pgoff.

    • If i > 0, then this is a superpage translation and pa.ppn[i − 1 : 0] = va.vpn[i − 1 : 0].

    • pa.ppn[LEVELS − 1 : i] = pte.ppn[LEVELS − 1 : i].

4.4 Sv39: Page-Based 39-bit Virtual-Memory System

This section describes a simple paged virtual-memory system designed for RV64 systems, which supports 39-bit virtual address spaces. The design of Sv39 follows the overall scheme of Sv32, and this section details only the differences between the schemes.

We specified multiple virtual memory systems for RV64 to relieve the tension between providing a large address space and minimizing address-translation cost. For many systems, 512 GiB of virtual-address space is ample, and so Sv39 suffices. Sv48 increases the virtual address space to 256 TiB, but increases the physical memory capacity dedicated to page tables, the latency of page-table traversals, and the size of hardware structures that store virtual addresses.

4.4.1 Addressing and Memory Protection

Sv39 implementations support a 39-bit virtual address space, divided into 4 KiB pages. An Sv39 address is partitioned as shown in Figure 1.19. Instruction fetch addresses and load and store effective addresses, which are 64 bits, must have bits 63–39 all equal to bit 38, or else a page-fault exception will occur. The 27-bit VPN is translated into a 44-bit PPN via a three-level page table, while the 12-bit page offset is untranslated.

When mapping between narrower and wider addresses, RISC-V usually zero-extends a narrower address to a wider size. The mapping between 64-bit virtual addresses and the 39-bit usable address space of Sv39 is not based on zero-extension but instead follows an entrenched convention that allows an OS to use one or a few of the most-significant bits of a full-size (64-bit) virtual address to quickly distinguish user and supervisor address regions.

Sv39 virtual address.
Sv39 physical address.
Sv39 page table entry.

Sv39 page tables contain 29 page table entries (PTEs), eight bytes each. A page table is exactly the size of a page and must always be aligned to a page boundary. The physical page number of the root page table is stored in the satp register’s PPN field.

The PTE format for Sv39 is shown in Figure 1.21. Bits 9–0 have the same meaning as for Sv32. Bits 63–54 are reserved for future standard use and must be zeroed by software for forward compatibility.

We reserved several PTE bits for a possible extension that improves support for sparse address spaces by allowing page-table levels to be skipped, reducing memory usage and TLB refill latency. These reserved bits may also be used to facilitate research experimentation. The cost is reducing the physical address space, but 64 PiB is presently ample. When it no longer suffices, the reserved bits that remain unallocated could be used to expand the physical address space.

Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv39 supports 2 MiB megapages and 1 GiB gigapages, each of which must be virtually and physically aligned to a boundary equal to its size. A page-fault exception is raised if the physical address is insufficiently aligned.

The algorithm for virtual-to-physical address translation is the same as in Section 1.3.2, except LEVELS equals 3 and PTESIZE equals 8.

4.5 Sv48: Page-Based 48-bit Virtual-Memory System

This section describes a simple paged virtual-memory system designed for RV64 systems, which supports 48-bit virtual address spaces. Sv48 is intended for systems for which a 39-bit virtual address space is insufficient. It closely follows the design of Sv39, simply adding an additional level of page table, and so this chapter only details the differences between the two schemes.

Implementations that support Sv48 must also support Sv39.

Systems that support Sv48 can also support Sv39 at essentially no cost, and so should do so to maintain compatibility with supervisor software that assumes Sv39.

4.5.1 Addressing and Memory Protection

Sv48 implementations support a 48-bit virtual address space, divided into 4 KiB pages. An Sv48 address is partitioned as shown in Figure 1.22. Instruction fetch addresses and load and store effective addresses, which are 64 bits, must have bits 63–48 all equal to bit 47, or else a page-fault exception will occur. The 36-bit VPN is translated into a 44-bit PPN via a four-level page table, while the 12-bit page offset is untranslated.

Sv48 virtual address.
Sv48 physical address.
Sv48 page table entry.

The PTE format for Sv48 is shown in Figure 1.24. Bits 9–0 have the same meaning as for Sv32. Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv48 supports 2 MiB megapages, 1 GiB gigapages, and 512 GiB terapages, each of which must be virtually and physically aligned to a boundary equal to its size. A page-fault exception is raised if the physical address is insufficiently aligned.

The algorithm for virtual-to-physical address translation is the same as in Section 1.3.2, except LEVELS equals 4 and PTESIZE equals 8.