Re: s390x TCG migration failure

[Juan Quintela]

Nina Schoetterl-Glausch <nsg@linux.ibm.com> wrote:
> Hi,
>
> We're seeing failures running s390x migration kvm-unit-tests tests with TCG.

As this is tcg, could you tell the exact command that you are running?
Does it needs to be in s390x host, rigth?

$ time ./tests/qtest/migration-test
# random seed: R02S940c4f22abc48b14868566639d3d6c77
# Skipping test: s390x host with KVM is required
1..0

real    0m0.003s
user    0m0.002s
sys     0m0.001s


> 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.

What exception?

What do you mean here by control block header?

> 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.
>
> 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) {

If block is all zeros, then remaining_size should be zero, so always
smaller than max_size.

I don't really fully understand what is going here.

>          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.

Later, Juan.