Re: [PATCH v13 03/21] KVM: x86: Refresh CPUID on write to guest MSR_IA32_XSS

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 8/21/2025 9:30 PM, Chao Gao wrote:
> From: Yang Weijiang <weijiang.yang@intel.com>
> 
> Update CPUID.(EAX=0DH,ECX=1).EBX to reflect current required xstate size
> due to XSS MSR modification.
> CPUID(EAX=0DH,ECX=1).EBX reports the required storage size of all enabled
> xstate features in (XCR0 | IA32_XSS). The CPUID value can be used by guest
> before allocate sufficient xsave buffer.
> 
> Note, KVM does not yet support any XSS based features, i.e. supported_xss
> is guaranteed to be zero at this time.
> 
> Opportunistically return KVM_MSR_RET_UNSUPPORTED if guest CPUID doesn't
> enumerate it. Since KVM_MSR_RET_UNSUPPORTED takes care of host_initiated
> cases, drop the host_initiated check.

It looks most of this patch is introducing

   vcpu->arch.guest_supported_xss

and use it to guard the wrmsr of MSR_IA32_XSS instead of KVM global 
kvm_caps.supported_xss.

So it's better to split it into two patches?

> Suggested-by: Sean Christopherson <seanjc@google.com>
> Co-developed-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
> Reviewed-by: Chao Gao <chao.gao@intel.com>
> Tested-by: Mathias Krause <minipli@grsecurity.net>
> Tested-by: John Allen <john.allen@amd.com>
> Tested-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> ---
>   arch/x86/include/asm/kvm_host.h |  3 ++-
>   arch/x86/kvm/cpuid.c            | 15 ++++++++++++++-
>   arch/x86/kvm/x86.c              |  9 +++++----
>   3 files changed, 21 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 0d3cc0fc27af..b7f82a421718 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 guest_supported_xss;
> +	u64 ia32_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..b5f87254ced7 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,
> @@ -305,7 +316,8 @@ static void kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu)
>   	best = kvm_find_cpuid_entry_index(vcpu, 0xD, 1);
>   	if (best && (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
>   		     cpuid_entry_has(best, X86_FEATURE_XSAVEC)))
> -		best->ebx = xstate_required_size(vcpu->arch.xcr0, true);
> +		best->ebx = xstate_required_size(vcpu->arch.xcr0 |
> +						 vcpu->arch.ia32_xss, true);
>   }
>   
>   static bool kvm_cpuid_has_hyperv(struct kvm_vcpu *vcpu)
> @@ -424,6 +436,7 @@ 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 569583943779..75b7a29721bb 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4011,16 +4011,17 @@ 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;
> +		if (vcpu->arch.ia32_xss == data)
> +			break;
>   		vcpu->arch.ia32_xss = data;
>   		vcpu->arch.cpuid_dynamic_bits_dirty = true;
>   		break;