KVM: SVM: Disable interception of SPEC_CTRL iff the MSR exists for the guest

JIRA: https://issues.redhat.com/browse/RHEL-47242

commit 674ffc6
Author: Sean Christopherson <seanjc@google.com>
Date:   Tue Jun 10 15:57:06 2025 -0700

    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: d00b99c ("KVM: SVM: Add support for Virtual SPEC_CTRL")
    Reported-by: Chao Gao <chao.gao@intel.com>
    Link: https://lore.kernel.org/r/20250610225737.156318-2-seanjc@google.com
    Signed-off-by: Sean Christopherson <seanjc@google.com>

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>

Author: Maxim Levitsky

arch/x86/kvm/svm/svm.c

@@ -1357,11 +1357,14 @@ 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);