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

[Yang, Weijiang]

On 1/27/2023 3:50 AM, Sean Christopherson wrote:
> 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@xxxxxxxxxxxxxxx>
> > Signed-off-by: Zhang Yi Z <yi.z.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > ---
> >   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.


Thanks for review!

Sure, I'll remove it.



> > +		}
> > +	}
> >   
> >   	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