Re: [PATCH v2 07/21] KVM:x86: Refresh CPUID on write to guest MSR_IA32_XSS

[Binbin Wu]

On 4/21/2023 9:46 PM, Yang Weijiang wrote:
> Update CPUID(EAX=0DH,ECX=1) when the guest's XSS is modified.
> CPUID(EAX=0DH,ECX=1).EBX reports current required storage size for all
> features enabled via XCR0 | XSS so that guest can allocate correct xsave
> buffer.
>
> Note, KVM does not yet support any XSS based features, i.e. supported_xss
> is guaranteed to be zero at this time.
>
> Co-developed-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> ---
>   arch/x86/kvm/cpuid.c | 11 ++++++++---
>   arch/x86/kvm/x86.c   |  6 ++++--
>   2 files changed, 12 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 123bf8b97a4b..dd6d5150d86a 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -276,9 +276,14 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
>   		best->ebx = xstate_required_size(vcpu->arch.xcr0, false);
>   
>   	best = cpuid_entry2_find(entries, nent, 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);
> +	if (best) {
> +		if (cpuid_entry_has(best, X86_FEATURE_XSAVES) ||
> +		    cpuid_entry_has(best, X86_FEATURE_XSAVEC))  {

Is it necessary to change to use two ifs?


> +			u64 xstate = vcpu->arch.xcr0 | vcpu->arch.ia32_xss;
> +
> +			best->ebx = xstate_required_size(xstate, true);
> +		}
> +	}
>   
>   	best = __kvm_find_kvm_cpuid_features(vcpu, entries, nent);
>   	if (kvm_hlt_in_guest(vcpu->kvm) && best &&
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 33a780fe820b..ab3360a10933 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3776,8 +3776,10 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
>   		 */
>   		if (data & ~kvm_caps.supported_xss)
>   			return 1;
> -		vcpu->arch.ia32_xss = data;
> -		kvm_update_cpuid_runtime(vcpu);
> +		if (vcpu->arch.ia32_xss != data) {
> +			vcpu->arch.ia32_xss = data;
> +			kvm_update_cpuid_runtime(vcpu);
> +		}
>   		break;
>   	case MSR_SMI_COUNT:
>   		if (!msr_info->host_initiated)