Re: [PATCH v15 06/41] KVM: x86: Check XSS validity against guest CPUIDs

[Binbin Wu]

On 9/13/2025 7:22 AM, Sean Christopherson wrote:
> From: Chao Gao <chao.gao@xxxxxxxxx>
>
> Maintain per-guest valid XSS bits and check XSS validity against them
> rather than against KVM capabilities. This is to prevent bits that are
> supported by KVM but not supported for a guest from being set.
>
> Opportunistically return KVM_MSR_RET_UNSUPPORTED on IA32_XSS MSR accesses
> if guest CPUID doesn't enumerate X86_FEATURE_XSAVES. Since
> KVM_MSR_RET_UNSUPPORTED takes care of host_initiated cases, drop the
> host_initiated check.
>
> Signed-off-by: Chao Gao <chao.gao@xxxxxxxxx>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

> ---
>   arch/x86/include/asm/kvm_host.h |  3 ++-
>   arch/x86/kvm/cpuid.c            | 12 ++++++++++++
>   arch/x86/kvm/x86.c              |  7 +++----
>   3 files changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 2762554cbb7b..d931d72d23c9 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -815,7 +815,6 @@ struct kvm_vcpu_arch {
>   	bool at_instruction_boundary;
>   	bool tpr_access_reporting;
>   	bool xfd_no_write_intercept;
> -	u64 ia32_xss;
>   	u64 microcode_version;
>   	u64 arch_capabilities;
>   	u64 perf_capabilities;
> @@ -876,6 +875,8 @@ struct kvm_vcpu_arch {
>   
>   	u64 xcr0;
>   	u64 guest_supported_xcr0;
> +	u64 ia32_xss;
> +	u64 guest_supported_xss;
>   
>   	struct kvm_pio_request pio;
>   	void *pio_data;
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index ad6cadf09930..46cf616663e6 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -263,6 +263,17 @@ static u64 cpuid_get_supported_xcr0(struct kvm_vcpu *vcpu)
>   	return (best->eax | ((u64)best->edx << 32)) & kvm_caps.supported_xcr0;
>   }
>   
> +static u64 cpuid_get_supported_xss(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_cpuid_entry2 *best;
> +
> +	best = kvm_find_cpuid_entry_index(vcpu, 0xd, 1);
> +	if (!best)
> +		return 0;
> +
> +	return (best->ecx | ((u64)best->edx << 32)) & kvm_caps.supported_xss;
> +}
> +
>   static __always_inline void kvm_update_feature_runtime(struct kvm_vcpu *vcpu,
>   						       struct kvm_cpuid_entry2 *entry,
>   						       unsigned int x86_feature,
> @@ -424,6 +435,7 @@ void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
>   	}
>   
>   	vcpu->arch.guest_supported_xcr0 = cpuid_get_supported_xcr0(vcpu);
> +	vcpu->arch.guest_supported_xss = cpuid_get_supported_xss(vcpu);
>   
>   	vcpu->arch.pv_cpuid.features = kvm_apply_cpuid_pv_features_quirk(vcpu);
>   
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 3b4258b38ad8..5a5af40c06a9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3984,15 +3984,14 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		}
>   		break;
>   	case MSR_IA32_XSS:
> -		if (!msr_info->host_initiated &&
> -		    !guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
> -			return 1;
> +		if (!guest_cpuid_has(vcpu, X86_FEATURE_XSAVES))
> +			return KVM_MSR_RET_UNSUPPORTED;
>   		/*
>   		 * KVM supports exposing PT to the guest, but does not support
>   		 * IA32_XSS[bit 8]. Guests have to use RDMSR/WRMSR rather than
>   		 * XSAVES/XRSTORS to save/restore PT MSRs.
>   		 */
> -		if (data & ~kvm_caps.supported_xss)
> +		if (data & ~vcpu->arch.guest_supported_xss)
>   			return 1;
>   		vcpu->arch.ia32_xss = data;
>   		vcpu->arch.cpuid_dynamic_bits_dirty = true;