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

[Sean Christopherson <seanjc@google.com>]

On Fri, Apr 23, 2021, Paolo Bonzini wrote:
> 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:

Too fast!  The below won't compile (s/msr_info/msr and 'r' needs to be defined),
and the get_msr() path needs the guest_cpuid_has() check.  I'll spin a v3. 

> 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
>