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

[Jim Mattson]

On Wed, Jul 15, 2026 at 10:45 AM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
>
> On Tue, Jul 14, 2026 at 11:32 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
> >
> > On Tue, Jul 14, 2026 at 4:41 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> >
> > > What complications?  Advertise the lowest common feature set for the pool, just
> > > like userspace has to do for literally every other feature.
> >
> > The problem with passing through the number of host ASIDs to the guest
> > is what to do when the combined host plus guest usage exceeds the
> > number of available ASIDs, particularly when FlushByASID is not
> > available.
>
> I think that particular scenario is fine because they are two separate
> domains. KVM assigns 1 ASIDs per vCPU for L2 (in this series),
> regardless of what L1 assigns L2. We flush that ASID if L1 changes the
> ASID it uses. We can use more than 1 ASID in the future, but I think
> KVM will still allocate a certain number of ASIDs per L2 and the
> domains will remain separate.

IIRC, Hyper-V uses two ASIDs per vCPU--or, at least, it used to. It
might be worth checking to see if that is still the case.

> If your concern is the fact that without FlushByASID KVM will do a lot
> of full flushes, you are right, and this series aggressively falls
> back to full flushes when FlushByASID isn't available, rather than
> using a different ASID like the current approach. My assumption has
> been that we don't care about performance that much on these old CPUs.

...until someone complains? :)