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

[Xiaoyao Li]

On Mon, 2019-03-18 at 09:38 -0700, Sean Christopherson wrote:
> 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.

I'll combine these two if statements into one.

I cannot understand why the WRMSR should only be done if the host's value is
non-zero. I think there is no depedency with host's value, if using the hardware
cpuid faulting. We just need to set the value to real hardware MSR.

> > +				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
> >