linux-kernel - [PATCH v2 01/32] KVM: SVM: Disable interception of SPEC

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Message-ID: <20250610225737.156318-2-seanjc@google.com>
Date: Tue, 10 Jun 2025 15:57:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Chao Gao <chao.gao@...el.com>, Borislav Petkov <bp@...en8.de>, Xin Li <xin@...or.com>, 
	Dapeng Mi <dapeng1.mi@...ux.intel.com>, Francesco Lavra <francescolavra.fl@...il.com>, 
	Manali Shukla <Manali.Shukla@....com>
Subject: [PATCH v2 01/32] KVM: SVM: Disable interception of SPEC_CTRL iff the
 MSR exists for the guest

Disable interception of SPEC_CTRL when the CPU virtualizes (i.e. context
switches) SPEC_CTRL if and only if the MSR exists according to the vCPU's
CPUID model.  Letting the guest access SPEC_CTRL is generally benign, but
the guest would see inconsistent behavior if KVM happened to emulate an
access to the MSR.

Fixes: d00b99c514b3 ("KVM: SVM: Add support for Virtual SPEC_CTRL")
Reported-by: Chao Gao <chao.gao@...el.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/svm/svm.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 0ad1a6d4fb6d..21e745acebc3 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -1362,11 +1362,14 @@ static void init_vmcb(struct kvm_vcpu *vcpu)
 	svm_recalc_instruction_intercepts(vcpu, svm);
 
 	/*
-	 * If the host supports V_SPEC_CTRL then disable the interception
-	 * of MSR_IA32_SPEC_CTRL.
+	 * If the CPU virtualizes MSR_IA32_SPEC_CTRL, i.e. KVM doesn't need to
+	 * manually context switch the MSR, immediately configure interception
+	 * of SPEC_CTRL, without waiting for the guest to access the MSR.
 	 */
 	if (boot_cpu_has(X86_FEATURE_V_SPEC_CTRL))
-		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SPEC_CTRL, 1, 1);
+		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SPEC_CTRL,
+				     guest_has_spec_ctrl_msr(vcpu),
+				     guest_has_spec_ctrl_msr(vcpu));
 
 	if (kvm_vcpu_apicv_active(vcpu))
 		avic_init_vmcb(svm, vmcb);
-- 
2.50.0.rc0.642.g800a2b2222-goog