On 6/11/2025 6:57 AM, Sean Christopherson wrote:
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@xxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
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));
Side topic, not related to this patch directly.
Setting to 1 for set_msr_interception() means to disable interception.
The name of the function seems a bit counterintuitive to me.
Maybe some description for the function can help people not familiar with
SVM code without further checking the implementation?
if (kvm_vcpu_apicv_active(vcpu))
avic_init_vmcb(svm, vmcb);
