Re: [PATCH 2/4] target/ppc: Ensure stcx size matches larx

["Nicholas Piggin" <npiggin@gmail.com>]

On Sun Jun 4, 2023 at 8:28 PM AEST, Nicholas Piggin wrote:
> Differently-sized larx/stcx. pairs can succeed if the starting address
> matches. Add a size check to require stcx. exactly match the larx that
> established the reservation.

Hmm, question: reserve_addr is a VMSTATE field, but reserve_val is not
(nor reserve_size after this patch).

Blue Swirl added that with commit a456d59c20f ("VM load/save support for
PPC CPU"), and when reserve_val was added in commit 18b21a2f83a
("target-ppc: retain l{w,d}arx loaded value") it did not get migrated.

Could we end up with reserve_addr != -1, but with a bogus reserve_val,
which could then permit a stcx. incorrectly? Not entirely outlandish if
reserve_val starts out initialised to zero.

Could we just clear the reserve in cpu_post_load? It is permitted to be
lost for an implementation-specific reason. Doesn't seem necessary to
try keep it alive over a migration.

Thanks,
Nick