Smpmp Extension

Versions

1.11.0
State

ratified

Ratification date

2019-12

Changes

  • Made the unused pmpaddr and pmpcfg fields WARL, rather than WIRI.

  • Specified semantics for PMP regions coarser than four bytes.

1.12.0
State

ratified

Ratification date

2021-12

Changes

  • PMP changes require an SFENCE.VMA on any hart that implements page-based virtual memory, even if VM is not currently enabled.

  • PMP reset values are now platform-defined.

  • An additional 48 optional PMP registers have been defined.

1.13.0
State

frozen

Synopsis

To support secure processing and contain faults, it is desirable to limit the physical addresses accessible by software running on a hart. The optional PMP (Physical Memory Protection) unit provides per-hart machine-mode control registers to allow physical memory access privileges (read, write, execute) to be specified for each physical memory region. The PMP values are checked in parallel with the PMA checks.

This is a placeholder extension name defined by the riscv-unified-db and isn’t considered an extension in the Privileged ISA manual. This was chosen as an extension rather than a M-mode parameter since the PMP has no visible ISA features (such as read-only-0 CSRs) if not present in an implementation. Making it an extension in the database prevents having the PMP CSRs show up in implementations that don’t have a PMP.

Parameters

This extension has the following implementation options:

NUM_PMP_ENTRIES

Type

integer

Valid Values

0 to 64

Description

Number of implemented PMP entries. Can be any value between 0-64, inclusive.

The architecture mandates that the number of implemented PMP registers must appear to be 0, 16, or 64.

Therefore, pmp registers will behave as follows according to NUN_PMP_ENTRIES:

NUM_PMP_ENTRIES

pmpaddr<0-15> / pmpcfg<0-3>

pmpaddr<16-63> / pmpcfg<4-15>

0

N

N

1-16

Y

N

17-64

Y

Y

  • N = Not implemented; access will cause IllegalInstruction if TRAP_ON_UNIMPLEMENTED_CSR is true

  • Y = Implemented; access will not cause an exception (from M-mode), but register may be read-only-zero if NUM_PMP_ENTRIES is less than the corresponding register

pmpcfgN for an odd N never exists when XLEN == 64

When NUM_PMP_ENTRIES is not exactly 0, 16, or 64, some extant pmp registers, and associated pmpNcfg, will be read-only zero (but will never cause an exception).
PMP_GRANULARITY

Type

integer

Valid Values

2 to 66

Description

log2 of the smallest supported PMP region.

Generally, for systems with an MMU, should not be smaller than 12, as that would preclude caching PMP results in the TLB along with virtual memory translations

Note that PMP_GRANULARITY is equal to G+2 (not G) as described in the privileged architecture.