KVM: x86: Tie Intel and AMD behavior for MSR_TSC_AUX to guest CPU model

Squish the Intel and AMD emulation of MSR_TSC_AUX together and tie it to
the guest CPU model instead of the host CPU behavior.  While not strictly
necessary to avoid guest breakage, emulating cross-vendor "architecture"
will provide consistent behavior for the guest, e.g. WRMSR fault behavior
won't change if the vCPU is migrated to a host with divergent behavior.

Note, the "new" kvm_is_supported_user_return_msr() checks do not add new
functionality on either SVM or VMX.  On SVM, the equivalent was
"tsc_aux_uret_slot < 0", and on VMX the check was buried in the
vmx_find_uret_msr() call at the find_uret_msr label.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210504171734.1434054-15-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1781,6 +1781,11 @@ int kvm_add_user_return_msr(u32 msr);
 int kvm_find_user_return_msr(u32 msr);
 int kvm_set_user_return_msr(unsigned index, u64 val, u64 mask);
 
+static inline bool kvm_is_supported_user_return_msr(u32 msr)
+{
+	return kvm_find_user_return_msr(msr) >= 0;
+}
+
 u64 kvm_scale_tsc(struct kvm_vcpu *vcpu, u64 tsc);
 u64 kvm_read_l1_tsc(struct kvm_vcpu *vcpu, u64 host_tsc);
 

arch/x86/kvm/svm/svm.c

@@ -2663,12 +2663,6 @@ static int svm_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 			msr_info->data |= (u64)svm->sysenter_esp_hi << 32;
 		break;
 	case MSR_TSC_AUX:
-		if (tsc_aux_uret_slot < 0)
-			return 1;
-		if (!msr_info->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_RDPID))
-			return 1;
 		msr_info->data = svm->tsc_aux;
 		break;
 	/*
@@ -2885,24 +2879,6 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		svm->sysenter_esp_hi = guest_cpuid_is_intel(vcpu) ? (data >> 32) : 0;
 		break;
 	case MSR_TSC_AUX:
-		if (tsc_aux_uret_slot < 0)
-			return 1;
-
-		if (!msr->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_RDPID))
-			return 1;
-
-		/*
-		 * Per Intel's SDM, bits 63:32 are reserved, but AMD's APM has
-		 * incomplete and conflicting architectural behavior.  Current
-		 * AMD CPUs completely ignore bits 63:32, i.e. they aren't
-		 * reserved and always read as zeros.  Emulate AMD CPU behavior
-		 * to avoid explosions if the vCPU is migrated from an AMD host
-		 * to an Intel host.
-		 */
-		data = (u32)data;
-
 		/*
 		 * TSC_AUX is usually changed only during boot and never read
 		 * directly.  Intercept TSC_AUX instead of exposing it to the

arch/x86/kvm/vmx/vmx.c

@@ -1981,12 +1981,6 @@ static int vmx_get_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		else
 			msr_info->data = vmx->pt_desc.guest.addr_a[index / 2];
 		break;
-	case MSR_TSC_AUX:
-		if (!msr_info->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_RDPID))
-			return 1;
-		goto find_uret_msr;
 	case MSR_IA32_DEBUGCTLMSR:
 		msr_info->data = vmcs_read64(GUEST_IA32_DEBUGCTL);
 		break;
@@ -2302,15 +2296,6 @@ static int vmx_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		else
 			vmx->pt_desc.guest.addr_a[index / 2] = data;
 		break;
-	case MSR_TSC_AUX:
-		if (!msr_info->host_initiated &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) &&
-		    !guest_cpuid_has(vcpu, X86_FEATURE_RDPID))
-			return 1;
-		/* Check reserved bit, higher 32 bits should be zero */
-		if ((data >> 32) != 0)
-			return 1;
-		goto find_uret_msr;
 	case MSR_IA32_PERF_CAPABILITIES:
 		if (data && !vcpu_to_pmu(vcpu)->version)
 			return 1;

arch/x86/kvm/x86.c

@@ -1642,6 +1642,30 @@ static int __kvm_set_msr(struct kvm_vcpu *vcpu, u32 index, u64 data,
 		 * invokes 64-bit SYSENTER.
 		 */
 		data = get_canonical(data, vcpu_virt_addr_bits(vcpu));
+		break;
+	case MSR_TSC_AUX:
+		if (!kvm_is_supported_user_return_msr(MSR_TSC_AUX))
+			return 1;
+
+		if (!host_initiated &&
+		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) &&
+		    !guest_cpuid_has(vcpu, X86_FEATURE_RDPID))
+			return 1;
+
+		/*
+		 * Per Intel's SDM, bits 63:32 are reserved, but AMD's APM has
+		 * incomplete and conflicting architectural behavior.  Current
+		 * AMD CPUs completely ignore bits 63:32, i.e. they aren't
+		 * reserved and always read as zeros.  Enforce Intel's reserved
+		 * bits check if and only if the guest CPU is Intel, and clear
+		 * the bits in all other cases.  This ensures cross-vendor
+		 * migration will provide consistent behavior for the guest.
+		 */
+		if (guest_cpuid_is_intel(vcpu) && (data >> 32) != 0)
+			return 1;
+
+		data = (u32)data;
+		break;
 	}
 
 	msr.data = data;
@@ -1678,6 +1702,18 @@ int __kvm_get_msr(struct kvm_vcpu *vcpu, u32 index, u64 *data,
 	if (!host_initiated && !kvm_msr_allowed(vcpu, index, KVM_MSR_FILTER_READ))
 		return KVM_MSR_RET_FILTERED;
 
+	switch (index) {
+	case MSR_TSC_AUX:
+		if (!kvm_is_supported_user_return_msr(MSR_TSC_AUX))
+			return 1;
+
+		if (!host_initiated &&
+		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP) &&
+		    !guest_cpuid_has(vcpu, X86_FEATURE_RDPID))
+			return 1;
+		break;
+	}
+
 	msr.index = index;
 	msr.host_initiated = host_initiated;