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
