Re: [PATCH v2 03/15] KVM: x86: Refresh CPUID on writes to MSR_IA32_XSS

[Sean Christopherson <seanjc@google.com>]

On Thu, Nov 24, 2022, Yang Weijiang wrote:
> Updated CPUID.0xD.0x1, which reports the current required storage size
> of all features enabled via XCR0 | XSS, when the guest's XSS is modified.
> 
> 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@linux.intel.com>
> Signed-off-by: Zhang Yi Z <yi.z.zhang@linux.intel.com>
> Signed-off-by: Yang Weijiang <weijiang.yang@intel.com>
> ---
>  arch/x86/kvm/cpuid.c | 16 +++++++++++++---
>  arch/x86/kvm/x86.c   |  6 ++++--
>  2 files changed, 17 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 6b5912578edd..85e3df6217af 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -272,9 +272,19 @@ 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))  {
> +			u64 xstate = vcpu->arch.xcr0 | vcpu->arch.ia32_xss;
> +
> +			best->ebx = xstate_required_size(xstate, true);
> +		}
> +
> +		if (!cpuid_entry_has(best, X86_FEATURE_XSAVES)) {
> +			best->ecx = 0;
> +			best->edx = 0;

ECX and EDX should be left alone, it is userspace's responsibility to provide a
sane CPUID model.  E.g. KVM doesn't clear EBX or EDX in CPUID.0xD.0x1 when XSAVE
is unsupported.

> +		}
> +	}
>  
>  	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 16726b44061b..888a153e32bc 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3685,8 +3685,10 @@ 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)
> -- 
> 2.27.0
>