M-mode software uses the MPT to create isolated physical memory regions associated with specific supervisor domains to enforce the required software access-control policies. Typically a supervisor domain has domain exclusive access to the memory region/pages of a region. Supervisor domains may also negotiate/grant access to a common/shared pool of memory regions/pages between supervisor domains - these memory regions may be accessible to all supervisor domains i.e. domain global-shared memory, or a specific subset of supervisor domains i.e., domain restricted-shared memory. Note here that the MPT enforces access-control based isolation only.
Some supervisor domain usages and threat models may also require cryptographic protection for memory (against forms of physical access attacks). In these cases, meta-data must be supplied to the platform as part of the physical memory accesses to identify the cryptographic context to be used for those physical memory accesses - this metadata may be identified via a function that combines the SDID and additional execution context fields e.g. VMID, ASID, to identify a unique cryptographic context. The scope of such cryptographic context identifiers are typically the SoC platform but may vary by implementations.
When accessing shared memory from a supervisor domain workload, it is required for the workload and/or the supervisor domain security manager to be in control of specifying the cryptographic contexts to be used for memory accesses - this context can be selected via the physical address metadata selector (Svpams). In the minimal case, globally-shared memory setup for all supervisor domains may require the selection of at least two cryptographic contexts for memory - domain exclusive and globally-shared memory. For domain restricted-shared cases, other n-lateral setup cryptographic contexts can be selected via the Svpams capability. Other meta-data values associated with physical addresses may also be selected via the Physical Address Metadata Selector.
The Svpams mechanism is independent of the Smmpt mechanism since Svpams enables additional properties to be enforced for memory accesses beyond the access-control/isolation mechanism that Smmpt enables.
Svpams may be enabled via the menvcfg (and henvcfg) registers. Once
enabled, the 4 bit PA metadata is derived as part of the address translation
and is used to select 1 of 16 possible metadata qualifiers that should be
applied to accesses that use the translated address. Any caching structures
that cache the address translation may also cache the PA meta-data as part of
the cached translation.
menvcfg and menvcfgh{reg: [
{bits: 1, name: 'FIOM', attr: '1'},
{bits: 3, name: 'WPRI', attr: '3'},
{bits: 2, name: 'CBIE', attr: '2'},
{bits: 1, name: 'CBCFE', attr: '1'},
{bits: 1, name: 'CBZE', attr: '1'},
{bits: 53, name: 'WPRI', attr: '53'},
{bits: 1, name: 'PAMSE', attr: '1'},
{bits: 1, name: 'PBMTE', attr: '1'},
{bits: 1, name: 'STCE', attr: '1'},
], config:{lanes: 2, hspace: 1700, bits: 64 }}
henvcfg and henvcfgh{reg: [
{bits: 1, name: 'FIOM'},
{bits: 3, name: 'WPRI'},
{bits: 2, name: 'CBIE'},
{bits: 1, name: 'CBCFE'},
{bits: 1, name: 'CBZE'},
{bits: 53, name: 'WPRI'},
{bits: 1, name: 'PAMSE'},
{bits: 1, name: 'PBMTE'},
{bits: 1, name: 'VSTCE'},
], config:{lanes: 2, hspace:1700, bits: 64}}
Sv57, Sv48, Sv39 with PAMS control field{reg: [
{bits: 1, name: 'V'},
{bits: 1, name: 'R'},
{bits: 1, name: 'W'},
{bits: 1, name: 'X'},
{bits: 1, name: 'U'},
{bits: 1, name: 'G'},
{bits: 1, name: 'A'},
{bits: 1, name: 'D'},
{bits: 2, name: 'Reserved'},
{bits: 43, name: 'PPN per Sv MODE'},
{bits: 4, name: 'PAMS'},
{bits: 4, name: 'Reserved'},
{bits: 2, name: 'PBMT'},
{bits: 1, name: 'N'},
], config:{lanes: 2, hspace:1700, bits: 64}}
Page table entry for Sv32 (TBD): Only option is to use bits 8:9 in the pte for a 2-bit PAMS
{reg: [
{bits: 4, name: 'TYPE'},
{bits: 60, name: 'Metadata (WARL) Interpreted per TYPE', attr: 'MXLEN-1'},
], config:{lanes: 1, hspace:1700, bits: 64}}
16 such CSRs are available from CSR0 - CSR15.
PAMS Types:
0 - None - Metadata is Reserved (must be zero).
1 - Encryption context identifier - Metadata is an encryption context
identfier (WARL).
Other metadata types may be defined in the future. Should the same CSRs be indexable from PMP also?
When two-stage translation is not enabled in a supervisor domain, and
satp.MODE is not zero, S-stage PAMS selects the physical address
metadata. When two-stage address translation is enabled using the H-extension
in a supervisor domain, If the hgatp.MODE is not zero, G-stage PAMS is
used to select the metadata and VS-stage PAMS is ignored (VS-stage PAMS
becomes reserved). This model is enforced to enforce the policy per the highest
privilege supervisor entity in the supervisor domain.