linux-kernel - [PATCH] kvm/x86: ARCH_CAPABILITIES should not be advertised on AMD

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Message-ID: <20250626125720.3132623-1-alexandre.chartre@oracle.com>
Date: Thu, 26 Jun 2025 14:57:20 +0200
From: Alexandre Chartre <alexandre.chartre@...cle.com>
To: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, pbonzini@...hat.com
Cc: seanjc@...gle.com, xiaoyao.li@...el.com, x86@...nel.org,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com,
        alexandre.chartre@...cle.com
Subject: [PATCH] kvm/x86: ARCH_CAPABILITIES should not be advertised on AMD

KVM emulates the ARCH_CAPABILITIES on x86 for both vmx and svm.
However the IA32_ARCH_CAPABILITIES MSR is an Intel-specific MSR
so it makes no sense to emulate it on AMD.

The AMD documentation specifies that this MSR is not defined on
the AMD architecture. So emulating this MSR on AMD can even cause
issues (like Windows BSOD) as the guest OS might not expect this
MSR to exist on such architecture.

Signed-off-by: Alexandre Chartre <alexandre.chartre@...cle.com>
---

A similar patch was submitted some years ago but it looks like it felt
through the cracks:
https://lore.kernel.org/kvm/20190307093143.77182-1-xiaoyao.li@linux.intel.com/

I am resurecting this change because some recent Windows updates (like OS Build
26100.4351) crashes on AMD KVM guests (BSOD with Stop code: UNSUPPORTED PROCESSOR)
just because the ARCH_CAPABILITIES is available.

---
 arch/x86/kvm/svm/svm.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index ab9b947dbf4f..600d2029156e 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5469,6 +5469,9 @@ static __init void svm_set_cpu_caps(void)

 	/* Don't advertise Bus Lock Detect to guest if SVM support is absent */
 	kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT);
+
+	/* Don't advertise ARCH_CAPABILITIES on AMD */
+	kvm_cpu_cap_clear(X86_FEATURE_ARCH_CAPABILITIES);
 }

 static __init int svm_hardware_setup(void)
-- 
2.43.5