Re: [PATCH v4 6/7] KVM: arm64: Relax locking for kvm_test_age_gfn and kvm_age_gfn

[James Houghton]

On Fri, May 31, 2024 at 12:18 PM Oliver Upton <oliver.upton@xxxxxxxxx> wrote:
>
> On Fri, May 31, 2024 at 12:11:33PM -0700, Oliver Upton wrote:
> > On Wed, May 29, 2024 at 06:05:09PM +0000, James Houghton wrote:
> >
> > [...]
> >
> > > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > > index 9e2bbee77491..eabb07c66a07 100644
> > > --- a/arch/arm64/kvm/hyp/pgtable.c
> > > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > > @@ -1319,10 +1319,8 @@ static int stage2_age_walker(const struct kvm_pgtable_visit_ctx *ctx,
> > >     data->young = true;
> > >
> > >     /*
> > > -    * stage2_age_walker() is always called while holding the MMU lock for
> > > -    * write, so this will always succeed. Nonetheless, this deliberately
> > > -    * follows the race detection pattern of the other stage-2 walkers in
> > > -    * case the locking mechanics of the MMU notifiers is ever changed.
> > > +    * This walk may not be exclusive; the PTE is permitted to change
> > > +    * from under us.
> > >      */
> > >     if (data->mkold && !stage2_try_set_pte(ctx, new))
> > >             return -EAGAIN;
> >
> > It is probably worth mentioning that if there was a race to update the
> > PTE then the GFN is most likely young, so failing to clear AF probably
> > isn't even consequential.

Thanks Oliver.

>
> Oh, and the WARN_ON() in kvm_pgtable_stage2_test_clear_young() is bogus
> now. Maybe demote it to:
>
>   r = kvm_pgtable_walk(...);
>   WARN_ON_ONCE(r && r != -EAGAIN);

Oh, indeed, thank you. Just to make sure -- does it make sense to
retry the cmpxchg if it fails? For example, the way I have it now for
x86[1], we retry the cmpxchg if the spte is still a leaf, otherwise we
move on to the next one having done nothing. Does something like that
make sense for arm64?

[1]: https://lore.kernel.org/linux-mm/20240529180510.2295118-6-jthoughton@xxxxxxxxxx/