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

[Jim Mattson]

On Wed, Jul 15, 2026 at 12:12 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
>
> On Wed, Jul 15, 2026 at 11:20 AM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
> >
> > 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? :)
>
> I am not sure how many people run VMs on pre-Bulldozer CPUs and care
> about performance, but practically it's only a problem if the full
> flush actually makes a difference. It is possible that the TLB is not
> big enough; flushing the ASID vs flushing the entire TLB makes a
> difference.

Well, the current implementation (one ASID per L2 per L1) already
flushes the entire TLB every time that L2 ASID is reused. Performance
can't get worse than that, I imagine.