Re: [PATCH v3 2/2] KVM: arm64: nv: Delay freeing of shadow S2 structures until VM destruction

On Sat, Aug 22, 2026 at 09:12:32AM +0100, Marc Zyngier wrote:
> On Fri, 21 Aug 2026 19:15:45 +0100,
> "Lorenzo Stoakes (ARM)" <[email protected]> wrote:
> > > @@ -1310,16 +1319,12 @@ void kvm_nested_s2_flush(struct kvm *kvm)
> > >
> > >  void kvm_arch_flush_shadow_all(struct kvm *kvm)
> > >  {
> > > -	for (int i = kvm->arch.nested_mmus_size - 1; i >= 0; i--) {
> > > +	for (int i = 0; i < kvm->arch.nested_mmus_size; i++) {
> >
> > Hmm why this was in reverse before? :) I guess some product of the
> > kvfree() bit or maybe something else?
>
> We allocate s2_mmus in S2_MMU_PER_VCPU chunks. Which means that it
> complicates the freeing of the these structures, as they can only be
> freed once all S2 PTs of that chunk have been freed. We have three
> options:
>
> - scan forward, and use complicated logic to work out that you have
>   freed the last PTs of a chunk, freeing with a negative offset from
>   the current point in the loop. Works, but hard to reason about in
>   fewer than 3 seconds.
>
> - scan backward, use simpler logic to ensure you have reached the
>   beginning of a chunk, nuke it.
>
> - have two loops, one for the PTs, one for the MMUs. That's what we
>   end-up with due to the different garbage collection phases.

Ah I see, so tear down page tables of the 2nd of the pair, then after doing it
for the 1st of the pair free the chunk, makes sense.

>
> Cheers,
>
> 	M.
>
> --
> Without deviation from the norm, progress is not possible.

--
Cheers, Lorenzo