Re: s390x TCG migration failure

[Thomas Huth]

On 24/03/2023 19.41, Nina Schoetterl-Glausch wrote:
> Hi,
>
> We're seeing failures running s390x migration kvm-unit-tests tests with TCG.
> Some initial findings:
> What seems to be happening is that after migration a control block header 
> accessed by the test code is all zeros which causes an unexpected exception.
> I did a bisection which points to c8df4a7aef ("migration: Split save_live_pending() 
> into state_pending_*") as the culprit.
> The migration issue persists after applying the fix e264705012 ("migration: I messed 
> state_pending_exact/estimate") on top of c8df4a7aef.

 Hi Nina,

could you please open a ticket in the QEMU bug tracker and add the "8.0" 
label there, so that it correctly shows up in the list of things that should 
be fixed before the 8.0 release?

> Applying
>
> diff --git a/migration/ram.c b/migration/ram.c
> index 56ff9cd29d..2dc546cf28 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -3437,7 +3437,7 @@ static void ram_state_pending_exact(void *opaque, 
> uint64_t max_size,
>   
>       uint64_t remaining_size = rs->migration_dirty_pages * TARGET_PAGE_SIZE;
>   
> -    if (!migration_in_postcopy()) {
> +    if (!migration_in_postcopy() && remaining_size < max_size) {
>           qemu_mutex_lock_iothread();
>           WITH_RCU_READ_LOCK_GUARD() {
>               migration_bitmap_sync_precopy(rs);
>
> on top fixes or hides the issue. (The comparison was removed by c8df4a7aef.)
> I arrived at this by experimentation, I haven't looked into why this makes a 
> difference.
>
> Any thoughts on the matter appreciated.

Juan, could you comment on this, please?

 Thomas