KVM: nSVM: Use KVM-governed feature framework to track "vVM{SAVE,LOAD} enabled"

Track "virtual VMSAVE/VMLOAD exposed to L1" via a governed feature flag
instead of using a dedicated bit/flag in vcpu_svm.

Opportunistically add a comment explaining why KVM disallows virtual
VMLOAD/VMSAVE when the vCPU model is Intel.

No functional change intended.

Link: https://lore.kernel.org/r/20230815203653.519297-11-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/governed_features.h

@@ -10,6 +10,7 @@ KVM_GOVERNED_X86_FEATURE(XSAVES)
 KVM_GOVERNED_X86_FEATURE(VMX)
 KVM_GOVERNED_X86_FEATURE(NRIPS)
 KVM_GOVERNED_X86_FEATURE(TSCRATEMSR)
+KVM_GOVERNED_X86_FEATURE(V_VMSAVE_VMLOAD)
 
 #undef KVM_GOVERNED_X86_FEATURE
 #undef KVM_GOVERNED_FEATURE

arch/x86/kvm/svm/nested.c

@@ -107,7 +107,7 @@ static void nested_svm_uninit_mmu_context(struct kvm_vcpu *vcpu)
 
 static bool nested_vmcb_needs_vls_intercept(struct vcpu_svm *svm)
 {
-	if (!svm->v_vmload_vmsave_enabled)
+	if (!guest_can_use(&svm->vcpu, X86_FEATURE_V_VMSAVE_VMLOAD))
 		return true;
 
 	if (!nested_npt_enabled(svm))

arch/x86/kvm/svm/svm.c

@@ -1194,8 +1194,6 @@ static inline void init_vmcb_after_set_cpuid(struct kvm_vcpu *vcpu)
 
 		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SYSENTER_EIP, 0, 0);
 		set_msr_interception(vcpu, svm->msrpm, MSR_IA32_SYSENTER_ESP, 0, 0);
-
-		svm->v_vmload_vmsave_enabled = false;
 	} else {
 		/*
 		 * If hardware supports Virtual VMLOAD VMSAVE then enable it
@@ -4266,7 +4264,13 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
 
 	svm->lbrv_enabled = lbrv && guest_cpuid_has(vcpu, X86_FEATURE_LBRV);
 
-	svm->v_vmload_vmsave_enabled = vls && guest_cpuid_has(vcpu, X86_FEATURE_V_VMSAVE_VMLOAD);
+	/*
+	 * Intercept VMLOAD if the vCPU mode is Intel in order to emulate that
+	 * VMLOAD drops bits 63:32 of SYSENTER (ignoring the fact that exposing
+	 * SVM on Intel is bonkers and extremely unlikely to work).
+	 */
+	if (!guest_cpuid_is_intel(vcpu))
+		kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_V_VMSAVE_VMLOAD);
 
 	svm->pause_filter_enabled = kvm_cpu_cap_has(X86_FEATURE_PAUSEFILTER) &&
 			guest_cpuid_has(vcpu, X86_FEATURE_PAUSEFILTER);

arch/x86/kvm/svm/svm.h

@@ -259,7 +259,6 @@ struct vcpu_svm {
 	bool soft_int_injected;
 
 	/* optional nested SVM features that are enabled for this guest  */
-	bool v_vmload_vmsave_enabled      : 1;
 	bool lbrv_enabled                 : 1;
 	bool pause_filter_enabled         : 1;
 	bool pause_threshold_enabled      : 1;