Re: [PATCH v3 11/27] KVM: SVM: Only flush the fallback ASID when used by a different vCPU

[Jim Mattson]

On Thu, Oct 15, 2026 at 3:03 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
>
> On Thu, Oct 15, 2026 at 2:44 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
> >
> > On Mon, Oct 12, 2026 at 4:36 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> > >
> > > Instead of always flushing the fallback ASID on every vCPU run, only
> > > flush it if the last vCPU that used it on the pCPU changes. This avoids
> > > constant TLB flushing on a vCPU using the fallback ASID if no other
> > > vCPUs that use the same ASID ran on the same pCPU.
> > >
> > > Note that checking (and flushing) the current ASID on vCPU load is not
> > > sufficient, as it is possible that a vCPU is using the fallback ASID
> > > only for L1 or only for L2. To do this correctly on vCPU load, both L1
> > > and L2 ASIDs would need to be checked, and the respective VMCB would
> > > need to be flushed. This would not be much simpler, and would perform
> > > unnecessary flushes in some cases (e.g.  loading a vCPU that uses the
> > > fallback ASID for L2, but L2 doesn't actually run until the vCPU is
> > > unloaded).
> > >
> > > Another alternative is always flushing on vCPU load if the current ASID
> > > is the fallback ASID, and flushing on nested transitions if the new ASID
> > > is the fallback ASID. Again, this is not much simpler and also incurs
> > > unnecessary flushes in some cases.
> > >
> > > Do the logical thing and track the vCPU using the fallback ASID instead.
> >
> > Wouldn't it be simpler to track the VMCB associated with the fallback
> > ASID on each pCPU? Then, you avoid any confusion regarding whether L1,
> > L2, or both have been assigned the fallback ASID.
>
> It would be if we went with checking the ASID on vCPU and nested
> transitions. But since we're checking in pre_svm_run(), I don't think
> it simplifies things. Oh, I guess it doesn't simplify this specific
> patch, but we don't need to check if both L1 and L2 use the fallback
> ASID on nested transitions.. but we have to check if they both use the
> same ASID anyway for SEV.
>
> OTOH, it would make cleanup more complicated, as we'd need to do it in
> both vCPU freeing and nested freeing.
>
> Did I miss something?

I trust your judgment.