On Thu, Oct 3, 2019 at 8:37 AM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:
>
> On 01/10/19 21:48, Jim Mattson wrote:
> > Does anyone know why kvm disallows enumerating INVPCID in the guest
> > CPUID when PCID is not enumerated? There are many far more nonsensical
> > CPUID combinations that kvm does allow, such as AVX512F without XSAVE,
> > or even PCID without LM. Why is INVPCID without PCID of paramount
> > concern?
> >
>
> I guess you're looking at this code:
>
> /* Exposing INVPCID only when PCID is exposed */
> bool invpcid_enabled =
> guest_cpuid_has(vcpu, X86_FEATURE_INVPCID) &&
> guest_cpuid_has(vcpu, X86_FEATURE_PCID);
>
> The INVPCID instruction will be disabled if !PCID && INVPCID, but it
> doesn't really disallow *enumerating* INVPCID. There is no particular
> reason for that, it was done like that originally ("KVM: VMX: Implement
> PCID/INVPCID for guests with EPT") and kept this way.
>
> With !PCID && INVPCID you could use PCID=0 operations as a fancy INVLPG,
> I suppose, but it is probably uninteresting enough that no one bothered
> changing it.
>
> Paolo
I was actually looking at the code a few lines lower:
if (!invpcid_enabled) {
exec_control &= ~SECONDARY_EXEC_ENABLE_INVPCID;
guest_cpuid_clear(vcpu, X86_FEATURE_INVPCID);
}
The call to guest_cpuid_clear *does* disallow enumerating INVPCID if
PCID isn't also enumerated. I'm just wondering why we bothered, since
we do so little sanitization of guest CPUID.
