Re: [PATCH v10 2/6] x86/sev: Initialize RMPOPT configuration MSRs

[Kalra, Ashish]

Hello Prateek,

On 7/21/2026 9:57 AM, K Prateek Nayak wrote:
> Hello Ashish,
> 
> On 7/21/2026 7:56 PM, Kalra, Ashish wrote:
>> That's why v7 moved to a runtime check. In v11 (as i mentioned) this will be a small local helper,  it will also handles two things
>> the cap-clear can't:
>>   - a contiguous (non-segmented) RMP, where RMPOPT isn't usable even with X86_FEATURE_RMPOPT set.
>>   - SNP being disabled at runtime (CC_ATTR_HOST_SEV_SNP).
>>
>>   static bool rmpopt_capable(void)
>>   {
>>         return cpu_feature_enabled(X86_FEATURE_RMPOPT) &&
>>                cc_platform_has(CC_ATTR_HOST_SEV_SNP) &&
> 
> CC_ATTR_HOST_SEV_SNP is set from bsp_determine_snp() ...
> 
>>                (rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED) &&
> 
> ... and that already checks snp_probe_rmptable_info() ...
> 
>>                rmp_segment_table;
> 
> .. and if iommu_snp_enable() fails snp_rmptable_init(), we clear
> CC_ATTR_HOST_SEV_SNP.
> 
> So, the way I see it, CC_ATTR_HOST_SEV_SNP being set means we have
> already tried provisioning RMP table.

Agreed, and that makes rmp_segment_table redundant — iommu_snp_enable() clears CC_ATTR_HOST_SEV_SNP when
snp_rmptable_init() fails, so the attr being set already implies rmp_segment_table != NULL. I'll drop it.

The one I'd keep is (rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED). It's a runtime config bit, not the CPUID feature:
setup_rmptable() allocates rmp_segment_table and keeps CC_ATTR_HOST_SEV_SNP set for both segmented and contiguous RMP, so
the MSR bit is the only thing that distinguishes them — and RMPOPT needs a segmented RMP.

Clearing X86_FEATURE_RMPOPT in snp_rmptable_init() won't work on its own as i mentioned in this reply earlier: it runs at
rootfs_initcall, after alternative_instructions(), so cpu_feature_enabled() keeps returning true off the already-patched
static_cpu_has().

So the helper becomes:

  static bool rmpopt_capable(void)
  {
        return cpu_feature_enabled(X86_FEATURE_RMPOPT) &&
               cc_platform_has(CC_ATTR_HOST_SEV_SNP) &&
               (rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED);
  }

Thanks,
Ashish

> 
> I'm assuming all platforms that have  X86_FEATURE_RMPOPT will all have
> segmented RMP. If not, you can just clear X86_FEATURE_RMPOPT in
> snp_rmptable_init() no?
> 
> We can never reach functions that need rmpopt_capable() without
> first checking CC_ATTR_HOST_SEV_SNP.
>