Re: [PATCH v2] KVM: SVM: Delay restoration of host MSR_TSC_AUX until return to userspace

[Paolo Bonzini]

On 22/04/21 22:12, Sean Christopherson wrote:
> 	case MSR_TSC_AUX:
> 		if (!boot_cpu_has(X86_FEATURE_RDTSCP))
> 			return 1;
>
> 		if (!msr_info->host_initiated &&
> 		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> 			return 1;
>
> 		/*
> 		 * TSC_AUX is usually changed only during boot and never read
> 		 * directly.  Intercept TSC_AUX instead of exposing it to the
> 		 * guest via direct_access_msrs, and switch it via user return.
> 		 */
> 		preempt_disable();
> 		r = kvm_set_user_return_msr(TSC_AUX_URET_SLOT, data, -1ull);
> 		preempt_enable();
> 		if (r)
> 			return 1;
>
> 		/*
> 		 * Bits 63:32 are dropped by AMD CPUs, but are reserved on
> 		 * Intel CPUs.  AMD's APM has incomplete and conflicting info
> 		 * on the architectural behavior; emulate current hardware as
> 		 * doing so ensures migrating from AMD to Intel won't explode.
> 		 */
> 		svm->tsc_aux = (u32)data;
> 		break;

Ok, squashed in the following:

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 14ff7f0963e9..00e9680969a2 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -2875,16 +2875,28 @@ static int svm_set_msr(struct kvm_vcpu *vcpu, struct msr_data *msr)
 		if (!boot_cpu_has(X86_FEATURE_RDTSCP))
 			return 1;
 
+		if (!msr_info->host_initiated &&
+		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
+			return 1;
+
 		/*
 		 * TSC_AUX is usually changed only during boot and never read
 		 * directly.  Intercept TSC_AUX instead of exposing it to the
 		 * guest via direct_access_msrs, and switch it via user return.
 		 */
-		svm->tsc_aux = data;
-
 		preempt_disable();
-		kvm_set_user_return_msr(TSC_AUX_URET_SLOT, data, -1ull);
+		r = kvm_set_user_return_msr(TSC_AUX_URET_SLOT, data, -1ull);
 		preempt_enable();
+		if (r)
+			return 1;
+
+		/*
+		 * Bits 63:32 are dropped by AMD CPUs, but are reserved on
+		 * Intel CPUs.  AMD's APM has incomplete and conflicting info
+		 * on the architectural behavior; emulate current hardware as
+		 * doing so ensures migrating from AMD to Intel won't explode.
+		 */
+		svm->tsc_aux = (u32)data;
 		break;
 	case MSR_IA32_DEBUGCTLMSR:
 		if (!boot_cpu_has(X86_FEATURE_LBRV)) {

Paolo