Re: [RFC PATCH] KVM: x86: Advertise PCID based on hardware support (with an asterisk)

[Paolo Bonzini]

On 4/11/24 18:31, Sean Christopherson wrote:
> Force set a synthetic feature, GUEST_PCID, if PCID can be safely used in
> virtual machines, even if the kernel itself disables PCID support, and
> advertise PCID support in KVM if GUEST_PCID is set.
>
> When running on a CPU that is affected by Intel's "Global INVLPG" erratum,
> which does NOT affect VMX non-root mode, it is safe to virtualize PCID for
> KVM guests, even though it is not safe for the kernel itself to enable PCID.
> Ditto for if the kernel disables PCID because CR4.PGE isn't supported.

But the guest would not use it if the f/m/s matches, right?  If the 
advantage is basically not splitting the migration pool, is that a 
concern for the affected Alder Lake/Gracemont/Raptor Lake processors?

Paolo