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

[Kalra, Ashish]

On 7/21/2026 9:46 PM, K Prateek Nayak wrote:
> Hello Ashish,
> 
> On 7/22/2026 2:16 AM, Kalra, Ashish wrote:
>> 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().
> 
> My bad! I actually meant in snp_probe_rmptable_info() during
> bsp_determine_snp(). Something like:
> 
> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index 8bcdce98f6dc..3ad53ef16c2f 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c
> @@ -684,10 +684,12 @@ bool snp_probe_rmptable_info(void)
>  	if (cpu_feature_enabled(X86_FEATURE_SEGMENTED_RMP))
>  		rdmsrq(MSR_AMD64_RMP_CFG, rmp_cfg);
>  
> -	if (rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED)
> +	if (rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED) {
>  		return probe_segmented_rmptable_info();
> -	else
> +	} else {
> +		setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
>  		return probe_contiguous_rmptable_info();
> +	}
>  }
>  
>  /*
> ---
> 
> Based on a quick look, it gets only called at bsp init and is early
> enough to allow patching alternatives and I don't see anything writing
> to MSR_AMD64_RMP_CFG.
> 
> Thoughts?
> 

Ah, that works — thanks.

With the contiguous case cleared early, X86_FEATURE_RMPOPT set now implies a segmented RMP, so the MSR-bit test drops out and the helper becomes:

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

I'll keep cc_platform_has(CC_ATTR_HOST_SEV_SNP) for the late failures the early clear can't cover: setup_segmented_rmptable() can still
fail in snp_rmptable_init() (rootfs_initcall, i.e. post-alternatives, where clearing the feature bit wouldn't stick) — and on that path
iommu_snp_enable() clears CC_ATTR_HOST_SEV_SNP, so the helper returns false. Same term also covers SNP being disabled at runtime.

Thanks,
Ashish