Re: [PATCH v3 13/27] KVM: nSVM: Add a placeholder ASID for L2

[Yosry Ahmed]

On Thu, Oct 15, 2026 at 2:55 PM Jim Mattson <jmattson@xxxxxxxxxx> wrote:
>
> On Mon, Oct 12, 2026 at 4:36 PM Yosry Ahmed <yosry@xxxxxxxxxx> wrote:
> >
> > In preparation for introducing a separate ASID for L2, introduce a
> > 'placeholder' ASID that is still the same as L1's ASID. This will
> > facilitate future changes that need to distinguish L1 and L2's ASIDs,
> > before actually using a different ASID for L1 and L2.
> >
> > On SEV VM migration, copy the new SEV ASID into asid02 and vmcb02, in
> > case nested was already initialized on the destination vCPU before
> > migration.
> >
> > No functional change intended.
> >
> > Signed-off-by: Yosry Ahmed <yosry@xxxxxxxxxx>
> > ---
> >  arch/x86/kvm/svm/nested.c | 5 +++--
> >  arch/x86/kvm/svm/sev.c    | 9 +++++++++
> >  arch/x86/kvm/svm/svm.h    | 2 ++
> >  3 files changed, 14 insertions(+), 2 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index bfb297b7d71ab..780132725bb79 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -701,7 +701,6 @@ static void nested_svm_transition_tlb_flush(struct kvm_vcpu *vcpu)
> >          *  - Honor L1's request to flush an ASID on nested VMRUN
> >          *  - Sync nested NPT MMU on VMRUN that flushes L2's ASID[*]
> >          *  - Don't crush a pending TLB flush in vmcb02 on nested VMRUN
> > -        *  - Flush L1's ASID on KVM_REQ_TLB_FLUSH_GUEST
>
> Um...How does this patch either do this TODO or render it moot?

L1 now has a separate ASID from L2, and it's the one we're already
flushing with KVM_REQ_TLB_FLUSH_GUEST. It's kinda already moot because
we already flush L1's ASID on KVM_REQ_TLB_FLUSH_GUEST.. or maybe it's
actually *wrong* because KVM_REQ_TLB_FLUSH_GUEST needs to flush the
current ASID, not always L1's ASID.

I can drop it separately if that helps.

>
> >          *
> >          * [*] Unlike nested EPT, SVM's ASID management can invalidate nested
> >          *     NPT guest-physical mappings on VMRUN.
>
> [ ... ]
>
> > +       /*
> > +        * If nested is already initialized on the destination vCPU, also update
> > +        * the nested ASID to match the new SEV ASID.
> > +        */
> > +       if (dst_svm->nested.vmcb02.ptr) {
> > +               dst_svm->nested.asid02 = asid;
> > +               dst_svm->nested.vmcb02.ptr->control.asid = asid;
>
> +1 to Sashiko.

Will do in the next version.