[PATCH v2 1/7] KVM: arm64: Move OUTSIDE_GUEST_MODE publication past context being saved

[Fuad Tabba]

Hi Marc,

On Tue, 29 Sep 2026 15:13:28 +0100, Marc Zyngier <maz at kernel.org> wrote:
[...]
> However, there is a problem if cmpxchg() fails, as there is no
> ordering in that case, and I'm not sure the smp_mb__before_atomic()
> saves the bacon in that case. It feels we'd need an acquire
> somewhere, a bit like this:

It doesn't save the bacon. I ran it through herd7 with the LKMM, the
fold walk in vgic_v3_fold_lr_state() against the list_del() in
vgic_flush_pending_lpis(): with smp_mb__before_atomic() and a failing
cmpxchg(), the walk can see the list_del(). With your
smp_load_acquire() it can't.

[...]
> We need a release so that we know for sure that any state stored
> before is visible by the time we can observe OUTSIDE_GUEST_MODE, and
> WRITE_ONCE() doesn't provide that (it can be reordered).
>
> I don't see what taking a lock changes to the ordering requirement.

The fold walk takes no lock, so the locks on the halt side don't order
it against the list_del(). herd7 agrees: WRITE_ONCE() with your
acquire can see the list_del() again.

[...]
> I don't understand what sashiko is trying to say, but this is clearly
> missing from the patch, see below. Not sure how I missed that one.

I don't understand Sashiko half of the time either :) That said, the
diff looks right to me.

Cheers,
/fuad