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

[Borislav Petkov]

On Mon, Nov 02, 2026 at 06:07:35PM -0600, Kalra, Ashish wrote:
> LGTM, tested on HW.

Ok, thanks.

This below is what I have now, and the code looks a lot better. If there are
no more complaints and it looks like what you tested, you can wait for me to
push out the two patches and then you can test tip:x86/urgent again.

Thx.

---
From: Ashish Kalra <ashish.kalra@xxxxxxx>
Date: Wed, 28 Oct 2026 19:49:22 +0000
Subject: [PATCH] x86/sev: Clear RMPOPT when segmented RMP is disabled by
 firmware

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.

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
   - Simplify snp_probe_rmptable_info()
   - Fix feature check order in snp_enable_rmpopt()
   - Add RMPOPT feature dependency on SEV_SNP ]

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
---
 arch/x86/kernel/cpu/cpuid-deps.c |  1 +
 arch/x86/virt/svm/sev.c          | 17 +++++------------
 2 files changed, 6 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/cpu/cpuid-deps.c b/arch/x86/kernel/cpu/cpuid-deps.c
index 99801e844b30..acbcec394ef1 100644
--- a/arch/x86/kernel/cpu/cpuid-deps.c
+++ b/arch/x86/kernel/cpu/cpuid-deps.c
@@ -93,6 +93,7 @@ static const struct cpuid_dep cpuid_deps[] = {
 	{ X86_FEATURE_SPEC_CTRL_SSBD,		X86_FEATURE_SPEC_CTRL },
 	{ X86_FEATURE_LASS,			X86_FEATURE_SMAP      },
 	{ X86_FEATURE_INVLPGB,			X86_FEATURE_PCID      },
+	{ X86_FEATURE_RMPOPT,			X86_FEATURE_SEV_SNP   },
 	{}
 };
 
diff --git a/arch/x86/virt/svm/sev.c b/arch/x86/virt/svm/sev.c
index bd70c4d0b774..417e6a41ac60 100644
--- a/arch/x86/virt/svm/sev.c
+++ b/arch/x86/virt/svm/sev.c
@@ -669,10 +669,10 @@ void snp_enable_rmpopt(void)
 	u64 base;
 	int cpu;
 
-	if (!cpu_feature_enabled(X86_FEATURE_RMPOPT))
+	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
 		return;
 
-	if (!cc_platform_has(CC_ATTR_HOST_SEV_SNP))
+	if (!cpu_feature_enabled(X86_FEATURE_RMPOPT))
 		return;
 
 	if (!rmpopt_soft_init)
@@ -819,18 +819,11 @@ 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 (probe_segmented_rmptable_info())
-				return true;
-
-			setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
-			return false;
-		}
-	} else {
-		setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
+		if (rmp_cfg & MSR_AMD64_SEG_RMP_ENABLED)
+			return probe_segmented_rmptable_info();
 	}
 
+	setup_clear_cpu_cap(X86_FEATURE_RMPOPT);
 	return probe_contiguous_rmptable_info();
 }
 
-- 
2.53.0

-- 
Regards/Gruss,
    Boris.

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