linux-kernel - Re: [PATCH v1 4/4] KVM: x86: Advertise support for the immediate form of MSR instructions

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Message-ID: <aIzRhGVgZXPXNwA1@google.com>
Date: Fri, 1 Aug 2025 07:39:00 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: "Xin Li (Intel)" <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, pbonzini@...hat.com, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, 
	dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com, 
	chao.gao@...el.com
Subject: Re: [PATCH v1 4/4] KVM: x86: Advertise support for the immediate form
 of MSR instructions

On Wed, Jul 30, 2025, Xin Li (Intel) wrote:
> Advertise support for the immediate form of MSR instructions to userspace
> if the instructions are supported by the underlying CPU.

SVM needs to explicitly clear the capability so that KVM doesn't over-advertise
support if AMD ever implements X86_FEATURE_MSR_IMM.

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index ca550c4fa174..7e7821ee8ee1 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -5311,8 +5311,12 @@ static __init void svm_set_cpu_caps(void)
        /* CPUID 0x8000001F (SME/SEV features) */
        sev_set_cpu_caps();
 
-       /* Don't advertise Bus Lock Detect to guest if SVM support is absent */
+       /*
+        * Clear capabilities that are automatically configured by common code,
+        * but that require explicit SVM support (that isn't yet implemented).
+        */
        kvm_cpu_cap_clear(X86_FEATURE_BUS_LOCK_DETECT);
+       kvm_cpu_cap_clear(X86_FEATURE_MSR_IMM);
 }
 
 static __init int svm_hardware_setup(void)