Re: [PATCH v3 07/21] KVM:x86: Refresh CPUID on write to guest MSR_IA32_XSS

[Chao Gao]

On Thu, May 11, 2023 at 12:08:43AM -0400, 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 | 7 +++++--
> arch/x86/kvm/x86.c   | 6 ++++--
> 2 files changed, 9 insertions(+), 4 deletions(-)
>
>diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>index 123bf8b97a4b..cbb1b8a65502 100644
>--- a/arch/x86/kvm/cpuid.c
>+++ b/arch/x86/kvm/cpuid.c
>@@ -277,8 +277,11 @@ static void __kvm_update_cpuid_runtime(struct kvm_vcpu *vcpu, struct kvm_cpuid_e
> 
> 	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);
>+		cpuid_entry_has(best, X86_FEATURE_XSAVEC))) {

Align indentation.

 	if (best && (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);
>+	}
> 
> 	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)

Shouldn't we check against the supported value of _this_ guest? similar to
guest_supported_xcr0.

> 			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
>