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migration/multifd: Fix compat with QEMU < 9.0 

Commit f5f48a7891 ("migration/multifd: Separate SYNC request with
normal jobs") changed the multifd source side to stop sending data
along with the MULTIFD_FLAG_SYNC, effectively introducing the concept
of a SYNC-only packet. Relying on that, commit d7e58f412c
("migration/multifd: Don't send ram data during SYNC") later came
along and skipped reading data from SYNC packets.

In a versions timeline like this:

  8.2 f5f48a7 9.0 9.1 d7e58f41 9.2

The issue arises that QEMUs < 9.0 still send data along with SYNC, but
QEMUs > 9.1 don't gather that data anymore. This leads to various
kinds of migration failures due to desync/missing data.

Stop checking for a SYNC packet on the destination and unconditionally
unfill the packet.

>From now on:

old -> new:
the source sends data + sync, destination reads normally

new -> new:
source sends only sync, destination reads zeros

new -> old:
source sends only sync, destination reads zeros

CC: qemu-stable@nongnu.org
Fixes: d7e58f412c ("migration/multifd: Don't send ram data during SYNC")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2720
Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
Message-Id: <20241213160120.23880-2-farosas@suse.de>
Signed-off-by: Fabiano Rosas <farosas@suse.de>
(cherry picked from commit b93d897ea2f0abbe7fc341a9ac176b5ecd0f3c93)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
This commit is contained in:
Fabiano Rosas 2024-12-13 13:01:19 -03:00 committed by Michael Tokarev
parent f5827eb0ac
commit 7e4480dde2
1 changed files with 9 additions and 6 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

15
migration/multifd.c
View File

@ -252,9 +252,8 @@ static int multifd_recv_unfill_packet(MultiFDRecvParams *p, Error **errp)
p->packet_num = be64_to_cpu(packet->packet_num);
p->packets_recved++;
if (!(p->flags & MULTIFD_FLAG_SYNC)) {
ret = multifd_ram_unfill_packet(p, errp);
}
/* Always unfill, old QEMUs (<9.0) send data along with SYNC */
ret = multifd_ram_unfill_packet(p, errp);
trace_multifd_recv_unfill(p->id, p->packet_num, p->flags,
p->next_packet_size);
@ -1151,9 +1150,13 @@ static void *multifd_recv_thread(void *opaque)
flags = p->flags;
/* recv methods don't know how to handle the SYNC flag */
p->flags &= ~MULTIFD_FLAG_SYNC;
if (!(flags & MULTIFD_FLAG_SYNC)) {
has_data = p->normal_num || p->zero_num;
}
/*
* Even if it's a SYNC packet, this needs to be set
* because older QEMUs (<9.0) still send data along with
* the SYNC packet.
*/
has_data = p->normal_num || p->zero_num;
qemu_mutex_unlock(&p->mutex);
} else {
/*