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

[K Prateek Nayak]

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?

-- 
Thanks and Regards,
Prateek