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

[Yosry Ahmed]

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.

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.