Re: [PATCH] KVM: x86: Give a hint when Win2016 might fail to boot due to XSAVES erratum

[Maciej S. Szmigiero]

On 26.01.2024 19:36, Paolo Bonzini wrote:
> On Wed, Jan 24, 2024 at 9:18 PM Maciej S. Szmigiero
> <mail@xxxxxxxxxxxxxxxxxxxxx> wrote:
> > +static void kvm_hv_xsaves_xsavec_maybe_warn_unlocked(struct kvm_vcpu *vcpu)
>
> Calling this function "unlocked" is confusing (others would say
> "locked" is confusing instead). The double-underscore convention is
> more common.
>
> > +{
> > +       struct kvm *kvm = vcpu->kvm;
> > +       struct kvm_hv *hv = to_kvm_hv(kvm);
> > +
> > +       if (hv->xsaves_xsavec_warned)
> > +               return;
> > +
> > +       if (!vcpu->arch.hyperv_enabled)
> > +               return;
>
> I think these two should be in kvm_hv_xsaves_xsavec_maybe_warn(),
> though the former needs to be checked again under the lock.
>
> > +       if ((hv->hv_guest_os_id & KVM_HV_WIN2016_GUEST_ID_MASK) !=
> > +           KVM_HV_WIN2016_GUEST_ID)
> > +               return;
>
> At this point there is no need to return. You can set
> xsaves_xsavec_warned and save the checks in the future.
>
> > +       /* UP configurations aren't affected */
> > +       if (atomic_read(&kvm->online_vcpus) < 2)
> > +               return;
> > +
> > +       if (boot_cpu_has(X86_FEATURE_XSAVES) ||
> > +           !guest_cpuid_has(vcpu, X86_FEATURE_XSAVEC))
> > +               return;
>
> boot_cpu_has can also be done first to cull the whole check.
>
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 27e23714e960..db0a2c40d749 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -1782,6 +1782,10 @@ static int set_efer
> >         if ((efer ^ old_efer) & KVM_MMU_EFER_ROLE_BITS)
> >                 kvm_mmu_reset_context(vcpu);
> >
> > +       if (guest_cpuid_is_amd_or_hygon(vcpu) &&
> > +           efer & EFER_SVME)
> > +               kvm_hv_xsaves_xsavec_maybe_warn(vcpu);
> > +
> >         return 0;
> > }
>
> Checking guest_cpuid_is_amd_or_hygon() is relatively expensive, it
> should be done after "efer & EFER_SVME" but really the bug can happen
> just as well on Intel as far as I understand? It's just less likely
> due to the AMD erratum.

Yes, I've checked this guest on an Intel host and it also fails to
boot in !XSAVES && XSAVEC configuration.

Only on Intel it's purely a theoretical problem as AFAIK there's
no corresponding Intel errata that disables just XSAVES.

> I'll send a v2.
>
> Paolo

Thanks,
Maciej