Re: [PATCH v4 0/8] Provide support for RMPREAD and a segmented RMP

["Nikunj A. Dadhania" <nikunj@amd.com>]

On 10/24/2024 12:11 AM, Tom Lendacky wrote:
> This series adds SEV-SNP support for a new instruction to read an RMP
> entry and for a segmented RMP table.
> 
> The RMPREAD instruction is used to return information related to an RMP
> entry in an architecturally defined format.
> 
> RMPREAD support is detected via CPUID 0x8000001f_EAX[21].
> 
> Segmented RMP support is a new way of representing the layout of an RMP
> table. Initial RMP table support required the RMP table to be contiguous
> in memory. RMP accesses from a NUMA node on which the RMP doesn't reside
> can take longer than accesses from a NUMA node on which the RMP resides.
> Segmented RMP support allows the RMP entries to be located on the same
> node as the memory the RMP is covering, potentially reducing latency
> associated with accessing an RMP entry associated with the memory. Each
> RMP segment covers a specific range of system physical addresses.
> 
> Segmented RMP support is detected and established via CPUID and MSRs.
> 
> CPUID:
>   - 0x8000001f_EAX[23]
>     - Indicates support for segmented RMP
> 
>   - 0x80000025_EAX
>     - [5:0]   : Minimum supported RMP segment size
>     - [11:6]  : Maximum supported RMP segment size
> 
>   - 0x80000025_EBX
>     - [9:0]   : Number of cacheable RMP segment definitions
>     - [10]    : Indicates if the number of cacheable RMP segments is
>                 a hard limit
> 
> MSR:
>   - 0xc0010132 (RMP_BASE)
>     - Is identical to current RMP support
> 
>   - 0xc0010133 (RMP_END)
>     - Should be in reset state if segmented RMP support is active
> 
>       For kernels that do not support segmented RMP, being in reset
>       state allows the kernel to disable SNP support if the non-segmented
>       RMP has not been allocated.
> 
>   - 0xc0010136 (RMP_CFG)
>     - [0]    : Indicates if segmented RMP is enabled
>     - [13:8] : Contains the size of memory covered by an RMP segment
>                (expressed as a power of 2)
> 
> The RMP segment size defined in the RMP_CFG MSR applies to all segments
> of the RMP. Therefore each RMP segment covers a specific range of system
> physical addresses. For example, if the RMP_CFG MSR value is 0x2401, then
> the RMP segment coverage value is 0x24 => 36, meaning the size of memory
> covered by an RMP segment is 64GB (1 << 36). So the first RMP segment
> covers physical addresses from 0 to 0xF_FFFF_FFFF, the second RMP segment
> covers physical addresses from 0x10_0000_0000 to 0x1F_FFFF_FFFF, etc.
> 
> When a segmented RMP is enabled, RMP_BASE points to the RMP bookkeeping
> area as it does today (16K in size). However, instead of RMP entries
> beginning immediately after the bookkeeping area, there is a 4K RMP
> segment table. Each entry in the table is 8-bytes in size:
> 
>   - [19:0]  : Mapped size (in GB)
>               The mapped size can be less than the defined segment size.
>               A value of zero, indicates that no RMP exists for the range
>               of system physical addresses associated with this segment.
>     [51:20] : Segment physical address
>               This address is left shift 20-bits (or just masked when
>               read) to form the physical address of the segment (1MB
>               alignment).
> 
> The series is based off of and tested against the tip tree:
>   https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git master
> 
>   94559bac4d40 ("Merge branch into tip/master: 'x86/sev'")
> 
> ---
> 
> Changes in v4:
> - Change the structure name of the newly introduced RMPREAD state data
>   to rmpread, to avoid churn around the renaming of the old rmpentry
>   structure.
> - Change the fam19h check to be explicit ZEN3/ZEN4 checks
> - Unify the use of u64 for RMP-related values instead of using a mix of
>   u64 and unsigned long.
> - Fix the RMP segment end calculation in __snp_fixup_e820_tables().
> - Minor message cleanups and code simplifications.
> 
> Changes in v3:
> - Added RMP documentation
> 
> Changes in v2:
> - Remove the self-describing check. The SEV firmware will ensure that
>   all RMP segments are covered by RMP entries.
> - Do not include RMP segments above maximum detected RAM address (64-bit
>   MMIO) in the system RAM coverage check.
> - Adjust new CPUID feature entries to match the change to how they are
>   or are not presented to userspace.
> 
> 
> Tom Lendacky (8):
>   x86/sev: Prepare for using the RMPREAD instruction to access the RMP
>   x86/sev: Add support for the RMPREAD instruction
>   x86/sev: Require the RMPREAD instruction after Zen4
>   x86/sev: Move the SNP probe routine out of the way
>   x86/sev: Map only the RMP table entries instead of the full RMP range
>   x86/sev: Treat the contiguous RMP table as a single RMP segment
>   x86/sev: Add full support for a segmented RMP table
>   x86/sev/docs: Document the SNP Reverse Map Table (RMP)
> 
>  .../arch/x86/amd-memory-encryption.rst        | 118 ++++
>  arch/x86/include/asm/cpufeatures.h            |   2 +
>  arch/x86/include/asm/msr-index.h              |   8 +-
>  arch/x86/kernel/cpu/amd.c                     |   9 +-
>  arch/x86/virt/svm/sev.c                       | 632 +++++++++++++++---
>  5 files changed, 675 insertions(+), 94 deletions(-)
> 

For the series:

Reviewed-by: Nikunj A Dadhania <nikunj@amd.com>

Regards,
Nikunj