Re: [PATCH v3 26/27] KVM: nSVM: Use different ASIDs for L1 and L2

[Yosry Ahmed]

On Mon, Oct 26, 2026 at 10:33 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Oct 22, 2026, Yosry Ahmed wrote:
> > On Thu, Oct 22, 2026 at 3:44 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > >
> > > On Fri, Oct 16, 2026, Jim Mattson wrote:
> > > > On Mon, Oct 12, 2026 at 4:37 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> > > > Moreover, does this change the semantics of PV TLB flush? In
> > > > record_steal_time(), we have:
> > > >
> > > > if (st_preempted & KVM_VCPU_FLUSH_TLB)
> > > >         kvm_vcpu_flush_tlb_guest(vcpu);
> > > >
> > > > IIUC, an idle vCPU that wakes into L2 will flush only L2's ASID. In
> > > > the past, with a shared ASID, L1's ASID was always flushed on emulated
> > > > VM-exit anyway. Now, L1's ASID will be unaffected. Though I can't find
> > > > any real documentation on PV TLB flush, it *seems* like the remote
> > > > vCPU would want L1's ASID to be flushed. OTOH, this change makes nSVM
> > > > behave just like nVMX.
> > >
> > > That's a bug that absolutely needs to be fixed, probably relatively urgently.  We
> > > should also document that passing through the steal_time shared information to L2
> > > to utilize PV TLB flushes for L2 is explicitly not supported.
> > >
> > > I hate to re-introduce deferred actions for L2 => L1 things, but that seems like
> > > the only sane way to handle this, because handling the !tdp_enabled case, where
> > > KVM needs to sync the current root, requires knowing what MMU is going to be in
> > > use when L1 regains control.  Yuck.
> >
> > Are you implying that *you* will be re-introducing deferred actions? :P
>
> Heh, probably not this calendar year.

Are you opposed to this series landing before that is fixed, keeping
in mind that it will expose nSVM to the pre-existing bug?