Re: [PATCH v16 10/10] KVM: x86: virtualize cpuid faulting

[Paolo Bonzini]

On 20/03/2017 09:16, Kyle Huey wrote:
> Hardware support for faulting on the cpuid instruction is not required to
> emulate it, because cpuid triggers a VM exit anyways. KVM handles the relevant
> MSRs (MSR_PLATFORM_INFO and MSR_MISC_FEATURES_ENABLE) and upon a
> cpuid-induced VM exit checks the cpuid faulting state and the CPL.
> kvm_require_cpl is even kind enough to inject the GP fault for us.
> 
> Signed-off-by: Kyle Huey <khuey@xxxxxxxxxxxx>
> Reviewed-by: David Matlack <dmatlack@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  2 ++
>  arch/x86/kvm/cpuid.c            |  3 +++
>  arch/x86/kvm/cpuid.h            | 11 +++++++++++
>  arch/x86/kvm/emulate.c          |  7 +++++++
>  arch/x86/kvm/x86.c              | 26 ++++++++++++++++++++++++++
>  5 files changed, 49 insertions(+)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 74ef58c8ff53..df0c2bd970a4 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -612,6 +612,8 @@ struct kvm_vcpu_arch {
>  	unsigned long dr7;
>  	unsigned long eff_db[KVM_NR_DB_REGS];
>  	unsigned long guest_debug_dr7;
> +	u64 msr_platform_info;
> +	u64 msr_misc_features_enables;
>  
>  	u64 mcg_cap;
>  	u64 mcg_status;
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index efde6cc50875..cb560a509041 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -876,6 +876,9 @@ int kvm_emulate_cpuid(struct kvm_vcpu *vcpu)
>  {
>  	u32 eax, ebx, ecx, edx;
>  
> +	if (cpuid_fault_enabled(vcpu) && !kvm_require_cpl(vcpu, 0))
> +		return;
> +
>  	eax = kvm_register_read(vcpu, VCPU_REGS_RAX);
>  	ecx = kvm_register_read(vcpu, VCPU_REGS_RCX);
>  	kvm_cpuid(vcpu, &eax, &ebx, &ecx, &edx);
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index 35058c2c0eea..a6fd40aade7c 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -205,4 +205,15 @@ static inline int guest_cpuid_stepping(struct kvm_vcpu *vcpu)
>  	return x86_stepping(best->eax);
>  }
>  
> +static inline bool supports_cpuid_fault(struct kvm_vcpu *vcpu)
> +{
> +	return vcpu->arch.msr_platform_info & MSR_PLATFORM_INFO_CPUID_FAULT;
> +}
> +
> +static inline bool cpuid_fault_enabled(struct kvm_vcpu *vcpu)
> +{
> +	return vcpu->arch.msr_misc_features_enables &
> +		  MSR_MISC_FEATURES_ENABLES_CPUID_FAULT;
> +}
> +
>  #endif
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 45c7306c8780..6a2ea945d01f 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -3854,6 +3854,13 @@ static int em_sti(struct x86_emulate_ctxt *ctxt)
>  static int em_cpuid(struct x86_emulate_ctxt *ctxt)
>  {
>  	u32 eax, ebx, ecx, edx;
> +	u64 msr = 0;
> +
> +	ctxt->ops->get_msr(ctxt, MSR_MISC_FEATURES_ENABLES, &msr);
> +	if (msr & MSR_MISC_FEATURES_ENABLES_CPUID_FAULT &&
> +	    ctxt->ops->cpl(ctxt)) {
> +		return emulate_gp(ctxt, 0);
> +	}
>  
>  	eax = reg_read(ctxt, VCPU_REGS_RAX);
>  	ecx = reg_read(ctxt, VCPU_REGS_RCX);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1faf620a6fdc..16d2082d85fb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -1008,6 +1008,8 @@ static u32 emulated_msrs[] = {
>  	MSR_IA32_MCG_CTL,
>  	MSR_IA32_MCG_EXT_CTL,
>  	MSR_IA32_SMBASE,
> +	MSR_PLATFORM_INFO,
> +	MSR_MISC_FEATURES_ENABLES,
>  };
>  
>  static unsigned num_emulated_msrs;
> @@ -2331,6 +2333,21 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  			return 1;
>  		vcpu->arch.osvw.status = data;
>  		break;
> +	case MSR_PLATFORM_INFO:
> +		if (!msr_info->host_initiated ||
> +		    data & ~MSR_PLATFORM_INFO_CPUID_FAULT ||
> +		    (!(data & MSR_PLATFORM_INFO_CPUID_FAULT) &&
> +		     cpuid_fault_enabled(vcpu)))
> +			return 1;
> +		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)))
> +			return 1;
> +		vcpu->arch.msr_misc_features_enables = data;
> +		break;
>  	default:
>  		if (msr && (msr == vcpu->kvm->arch.xen_hvm_config.msr))
>  			return xen_hvm_config(vcpu, data);
> @@ -2545,6 +2562,12 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>  			return 1;
>  		msr_info->data = vcpu->arch.osvw.status;
>  		break;
> +	case MSR_PLATFORM_INFO:
> +		msr_info->data = vcpu->arch.msr_platform_info;
> +		break;
> +	case MSR_MISC_FEATURES_ENABLES:
> +		msr_info->data = vcpu->arch.msr_misc_features_enables;
> +		break;
>  	default:
>  		if (kvm_pmu_is_valid_msr(vcpu, msr_info->index))
>  			return kvm_pmu_get_msr(vcpu, msr_info->index, &msr_info->data);
> @@ -7724,6 +7747,9 @@ void kvm_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	if (!init_event) {
>  		kvm_pmu_reset(vcpu);
>  		vcpu->arch.smbase = 0x30000;
> +
> +		vcpu->arch.msr_platform_info = MSR_PLATFORM_INFO_CPUID_FAULT;
> +		vcpu->arch.msr_misc_features_enables = 0;
>  	}
>  
>  	memset(vcpu->arch.regs, 0, sizeof(vcpu->arch.regs));
> 

Patch 10 applied, thanks.

Paolo