KVM: x86: virtualize cpuid faulting

Hardware support for faulting on the cpuid instruction is not required to
emulate it, because cpuid triggers a VM exit anyways. KVM handles the relevant
MSRs (MSR_PLATFORM_INFO and MSR_MISC_FEATURES_ENABLE) and upon a
cpuid-induced VM exit checks the cpuid faulting state and the CPL.
kvm_require_cpl is even kind enough to inject the GP fault for us.

Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Reviewed-by: David Matlack <dmatlack@google.com>
[Return "1" from kvm_emulate_cpuid, it's not void. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -611,6 +611,8 @@ struct kvm_vcpu_arch {
 	unsigned long dr7;
 	unsigned long eff_db[KVM_NR_DB_REGS];
 	unsigned long guest_debug_dr7;
+	u64 msr_platform_info;
+	u64 msr_misc_features_enables;
 
 	u64 mcg_cap;
 	u64 mcg_status;

arch/x86/kvm/cpuid.c

@@ -876,6 +876,9 @@ int kvm_emulate_cpuid(struct kvm_vcpu *vcpu)
 {
 	u32 eax, ebx, ecx, edx;
 
+	if (cpuid_fault_enabled(vcpu) && !kvm_require_cpl(vcpu, 0))
+		return 1;
+
 	eax = kvm_register_read(vcpu, VCPU_REGS_RAX);
 	ecx = kvm_register_read(vcpu, VCPU_REGS_RCX);
 	kvm_cpuid(vcpu, &eax, &ebx, &ecx, &edx);

arch/x86/kvm/cpuid.h

@@ -205,4 +205,15 @@ static inline int guest_cpuid_stepping(struct kvm_vcpu *vcpu)
 	return x86_stepping(best->eax);
 }
 
+static inline bool supports_cpuid_fault(struct kvm_vcpu *vcpu)
+{
+	return vcpu->arch.msr_platform_info & MSR_PLATFORM_INFO_CPUID_FAULT;
+}
+
+static inline bool cpuid_fault_enabled(struct kvm_vcpu *vcpu)
+{
+	return vcpu->arch.msr_misc_features_enables &
+		  MSR_MISC_FEATURES_ENABLES_CPUID_FAULT;
+}
+
 #endif

arch/x86/kvm/emulate.c

@@ -3854,6 +3854,13 @@ static int em_sti(struct x86_emulate_ctxt *ctxt)
 static int em_cpuid(struct x86_emulate_ctxt *ctxt)
 {
 	u32 eax, ebx, ecx, edx;
+	u64 msr = 0;
+
+	ctxt->ops->get_msr(ctxt, MSR_MISC_FEATURES_ENABLES, &msr);
+	if (msr & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT &&
+	    ctxt->ops->cpl(ctxt)) {
+		return emulate_gp(ctxt, 0);
+	}
 
 	eax = reg_read(ctxt, VCPU_REGS_RAX);
 	ecx = reg_read(ctxt, VCPU_REGS_RCX);

arch/x86/kvm/x86.c

@@ -1007,6 +1007,8 @@ static u32 emulated_msrs[] = {
 	MSR_IA32_MCG_CTL,
 	MSR_IA32_MCG_EXT_CTL,
 	MSR_IA32_SMBASE,
+	MSR_PLATFORM_INFO,
+	MSR_MISC_FEATURES_ENABLES,
 };
 
 static unsigned num_emulated_msrs;
@@ -2314,6 +2316,21 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			return 1;
 		vcpu->arch.osvw.status = data;
 		break;
+	case MSR_PLATFORM_INFO:
+		if (!msr_info->host_initiated ||
+		    data & ~MSR_PLATFORM_INFO_CPUID_FAULT ||
+		    (!(data & MSR_PLATFORM_INFO_CPUID_FAULT) &&
+		     cpuid_fault_enabled(vcpu)))
+			return 1;
+		vcpu->arch.msr_platform_info = data;
+		break;
+	case MSR_MISC_FEATURES_ENABLES:
+		if (data & ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT ||
+		    (data & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT &&
+		     !supports_cpuid_fault(vcpu)))
+			return 1;
+		vcpu->arch.msr_misc_features_enables = data;
+		break;
 	default:
 		if (msr && (msr == vcpu->kvm->arch.xen_hvm_config.msr))
 			return xen_hvm_config(vcpu, data);
@@ -2529,6 +2546,12 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			return 1;
 		msr_info->data = vcpu->arch.osvw.status;
 		break;
+	case MSR_PLATFORM_INFO:
+		msr_info->data = vcpu->arch.msr_platform_info;
+		break;
+	case MSR_MISC_FEATURES_ENABLES:
+		msr_info->data = vcpu->arch.msr_misc_features_enables;
+		break;
 	default:
 		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
 			return kvm_pmu_get_msr(vcpu, msr_info->index, &msr_info->data);
@@ -7710,6 +7733,9 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
 	if (!init_event) {
 		kvm_pmu_reset(vcpu);
 		vcpu->arch.smbase = 0x30000;
+
+		vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
+		vcpu->arch.msr_misc_features_enables = 0;
 	}
 
 	memset(vcpu->arch.regs, 0, sizeof(vcpu->arch.regs));