Re: [PATCH v2 2/2] kvm/vmx: Using hardware cpuid faulting to avoid emulation overhead

[Sean Christopherson]

On Mon, Mar 18, 2019 at 07:43:24PM +0800, Xiaoyao Li wrote:
> Current cpuid faulting of guest is purely emulated in kvm, which exploits
> CPUID vm exit to inject #GP to guest. However, if host hardware cpu has
> X86_FEATURE_CPUID_FAULT, we can just use the hardware cpuid faulting for
> guest to avoid the vm exit overhead.

Heh, I obviously didn't look at this patch before responding to patch 1/2.

> Note: cpuid faulting takes higher priority over CPUID instruction vm
> exit (Intel SDM vol3.25.1.1).
> 
> Since cpuid faulting only exists on some Intel's cpu, just apply this
> optimization to vmx.
> 
> Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 ++
>  arch/x86/kvm/vmx/vmx.c          | 19 +++++++++++++++----
>  arch/x86/kvm/x86.c              | 15 ++++++++++++---
>  3 files changed, 29 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index ce79d7bfe1fd..14cad587b804 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1339,6 +1339,8 @@ void kvm_lmsw(struct kvm_vcpu *vcpu, unsigned long msw);
>  void kvm_get_cs_db_l_bits(struct kvm_vcpu *vcpu, int *db, int *l);
>  int kvm_set_xcr(struct kvm_vcpu *vcpu, u32 index, u64 xcr);
>  
> +int kvm_supported_msr_misc_features_enables(struct kvm_vcpu *vcpu, u64 data);
> +
>  int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr);
>  int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr);
>  
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 2c59e0209e36..6b413e471dca 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1037,7 +1037,7 @@ static void pt_guest_exit(struct vcpu_vmx *vmx)
>  
>  static void vmx_save_host_cpuid_fault(struct vcpu_vmx *vmx)
>  {
> -	u64 host_val;
> +	u64 host_val, guest_val;
>  
>  	if (!boot_cpu_has(X86_FEATURE_CPUID_FAULT))
>  		return;
> @@ -1045,10 +1045,12 @@ static void vmx_save_host_cpuid_fault(struct vcpu_vmx *vmx)
>  	rdmsrl(MSR_MISC_FEATURES_ENABLES, host_val);
>  	vmx->host_msr_misc_features_enables = host_val;
>  
> -	/* clear cpuid fault bit to avoid it leak to guest */
> -	if (host_val & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT) {
> +	guest_val = vmx->vcpu.arch.msr_misc_features_enables;
> +
> +	/* we can use the hardware cpuid faulting to avoid emulation overhead */
> +	if ((host_val ^ guest_val) & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT) {
>  		wrmsrl(MSR_MISC_FEATURES_ENABLES,
> -		       host_val & ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT);
> +		       host_val ^ MSR_MISC_FEATURES_ENABLES_CPUID_FAULT);
>  	}
>  }
>  
> @@ -2057,6 +2059,15 @@ 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_MISC_FEATURES_ENABLES:
> +		if (!kvm_supported_msr_misc_features_enables(vcpu, data))
> +			return 1;
> +		if (boot_cpu_has(X86_FEATURE_CPUID_FAULT)) {
> +			if (vmx->loaded_cpu_state)

No need for two separate if statements.  And assuming we're checking the
existing shadow value when loading guest/host state, the WRMSR should
only be done if the host's value is non-zero.

> +				wrmsrl(MSR_MISC_FEATURES_ENABLES, data);
> +		}
> +		vcpu->arch.msr_misc_features_enables = data;
> +		break;
>  	case MSR_TSC_AUX:
>  		if (!msr_info->host_initiated &&
>  		    !guest_cpuid_has(vcpu, X86_FEATURE_RDTSCP))
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 434ec113cc79..33a8c95b2f2e 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2449,6 +2449,17 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
>  		&vcpu->arch.st.steal, sizeof(struct kvm_steal_time));
>  }
>  
> +int kvm_supported_msr_misc_features_enables(struct kvm_vcpu *vcpu, u64 data)
> +{
> +	if (data & ~MSR_MISC_FEATURES_ENABLES_CPUID_FAULT ||
> +	    (data & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT &&
> +	     !supports_cpuid_fault(vcpu)))
> +		return 0;
> +	else
> +		return 1;
> +}
> +EXPORT_SYMBOL_GPL(kvm_supported_msr_misc_features_enables);
> +
>  int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  {
>  	bool pr = false;
> @@ -2669,9 +2680,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  		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)))
> +		if (!kvm_supported_msr_misc_features_enables(vcpu, data))
>  			return 1;
>  		vcpu->arch.msr_misc_features_enables = data;
>  		break;
> -- 
> 2.19.1
>