Re: [RFC PATCH v2 02/25] KVM: SVM: Passthrough the number of supported ASIDs

[Jim Mattson]

On Thu, Jul 23, 2026 at 7:07 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Jul 23, 2026, Jim Mattson wrote:
> > However, since KVM completely ignores NASID, it would be sad if
> > migration were blocked because the target has a lower NASID value than
> > the source.
> >
> > That's an argument against changing it without some kind of userspace
> > opt-in. Increasing it seems safe, but if userspace just reflects the
> > value into KVM_SET_CPUID2, then an increase would block kernel
> > rollback.
> >
> > In any case, I think the new value should be a constant so it doesn't
> > imply a migration constraint. Perhaps we should adopt the highest
> > value reported on a physical CPU to date? I think that would be
> > Bulldozer's 0x10000.
>
> Or keep KVM's enumeration at 8, then find somewhere in KVM's documentation to
> describe KVM's behavior and provide guidance for userspace, but otherwise make
> it userspace's problem to solve?  I'd be a-ok with that.

That sounds best to me. Document that KVM ignores the guest NASID and
commits to always supporting 2^32 ASIDs. Advise userspace to populate
NASID with a power of 2, and perhaps mention that hardware has only
reported three different values over the last 20 years: 0x40, 0x10000,
and 0x8000.