Re: [PATCH v5] KVM: x86/xen: Update Xen CPUID Leaf 4 (tsc info) sub-leaves, if present

[Vitaly Kuznetsov]

Sean Christopherson <seanjc@xxxxxxxxxx> writes:

> On Tue, Jul 12, 2022, Durrant, Paul wrote:
>> > > @@ -1855,3 +1858,51 @@ void kvm_xen_destroy_vm(struct kvm *kvm)
>> > >       if (kvm->arch.xen_hvm_config.msr)
>> > >               static_branch_slow_dec_deferred(&kvm_xen_enabled);
>> > >  }
>> > > +
>> > > +void kvm_xen_after_set_cpuid(struct kvm_vcpu *vcpu)
>> > > +{
>> > > +     u32 base = 0;
>> > > +     u32 limit;
>> > > +     u32 function;
>> > > +
>> > > +     vcpu->arch.xen.cpuid_tsc_info = 0;
>> > > +
>> > > +     for_each_possible_hypervisor_cpuid_base(function) {
>> > > +             struct kvm_cpuid_entry2 *entry = kvm_find_cpuid_entry(vcpu, function, 0);
>> > > +
>> > > +             if (entry &&
>> > > +                 entry->ebx == XEN_CPUID_SIGNATURE_EBX &&
>> > > +                 entry->ecx == XEN_CPUID_SIGNATURE_ECX &&
>> > > +                 entry->edx == XEN_CPUID_SIGNATURE_EDX) {
>> > > +                     base = function;
>> > > +                     limit = entry->eax;
>> > > +                     break;
>> > > +             }
>> > > +     }
>> > > +     if (!base)
>> > > +             return;
>> > 
>> > Rather than open code a variant of kvm_update_kvm_cpuid_base(), that helper can
>> > be tweaked to take a signature.  Along with a patch to provide a #define for Xen's
>> > signature as a string, this entire function becomes a one-liner.
>> > 
>> 
>> Sure, but as said above, we could make capturing the limit part of the
>> general function too. It could even be extended to capture the Hyper-V
>> base/limit too.  As for defining the sig as a string... I guess it would be
>> neater to use the values from the Xen header, but it'll probably make the
>> code more ugly so a secondary definition is reasonable.
>
> The base needs to be captured separately for KVM and Xen because KVM (and presumably
> Xen itself since Xen also allows a variable base) supports advertising multiple
> hypervisors to the guest.  I don't know if there are any guests that will concurrently
> utilize multiple hypervisor's paravirt features, so maybe we could squeak by, but
> saving 4 bytes isn't worth the risk.
>
> AFAIK, Hyper-V doesn't allow for a variable base, and so doesn't utilize the
> for_each_possible... macro.

FWIW, this matches my understanding too: Windows guests don't seem to
check anything besides 0x40000001 and give up if Hyper-V's id is not
there.

-- 
Vitaly