Re: [PATCH v4 1/2] x86/sev: Clear RMPOPT when segmented RMP is disabled by firmware

[K Prateek Nayak]

Hello Boris,

On 11/2/2026 10:58 AM, Borislav Petkov wrote:
> Segmented RMP support is advertised via X86_FEATURE_SEGMENTED_RMP, and the
> firmware enables it via MSR_AMD64_RMP_CFG[SEG_RMP_ENABLED]. The latter can be
> cleared in platform BIOS by the user, in which case the system runs in
> contiguous RMP setting.

nit. In the latter case, SNP is disabled entirely in bsp_determine_snp and
the system won't continue with contiguous RMP.

> snp_probe_rmptable_info() does not clear X86_FEATURE_RMPOPT in the
> advertised-but-firmware-disabled case.
> 
> As a result, this does not prevent snp_enable_rmpopt() from trying to program
> MSR_AMD64_RMPOPT_BASE, which raises a #GP on the MSR access.
> 
> Make sure X86_FEATURE_RMPOPT is cleared in all cases where segmented RMP is
> not enabled.
> 
>   [ bp:
>    - Put the commit message on Ozempic
>    - Flip the logic in the function so that the segmented RMP disabled
>      case goes first and the flow gets simpler this way. ]
> 
> Fixes: 15d974be0a3b ("x86/sev: Initialize RMPOPT configuration MSRs")
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> Signed-off-by: Borislav Petkov (AMD) <bp@xxxxxxxxx>
> Link: https://patch.msgid.link/56da047d63b22f03a7e709fc172a5543da894d67.1793216372.git.ashish.kalra@xxxxxxx

Feel free to include:

Reviewed-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>

> ---
>  arch/x86/virt/svm/sev.c | 21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
> index bd70c4d0b774..ff6d17795054 100644
> --- a/arch/x86/virt/svm/sev.c
> +++ b/arch/x86/virt/svm/sev.c
> @@ -817,21 +817,20 @@ static bool probe_segmented_rmptable_info(void)
>  
>  bool snp_probe_rmptable_info(void)
>  {
> -	if (cpu_feature_enabled(X86_FEATURE_SEGMENTED_RMP)) {
> -		rdmsrq(MSR_AMD64_RMP_CFG, rmp_cfg);
> +	if (!cpu_feature_enabled(X86_FEATURE_SEGMENTED_RMP)) {
> +		setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
>  
> -		if (rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED) {
> -			if (probe_segmented_rmptable_info())
> -				return true;
> +		return probe_contiguous_rmptable_info();
> +	}
>  
> -			setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
> -			return false;
> -		}
> -	} else {
> -		setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
> +	rdmsrq(MSR_AMD64_RMP_CFG, rmp_cfg);
> +	if (rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED) {
> +		if (probe_segmented_rmptable_info())
> +			return true;
>  	}
>  
> -	return probe_contiguous_rmptable_info();
> +	setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
> +	return false;
>  }
>  
>  /*

-- 
Thanks and Regards,
Prateek