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migration: Make switchover-ack re-usable

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Switchover-ack is a mechanism to synchronize between source and
destination QEMU during migration to prevent the source from switching
over prematurely.

VFIO uses switchover-ack to ensure switchover happens only after
destination side has loaded the precopy initial bytes. This is important
for VFIO, as otherwise downtime could be impacted and be higher.

In its current state, switchover-ack is a one-time mechanism, meaning
that switchover is acked only once and past that another ACK cannot be
requested again. This was sufficient until now, as VFIO precopy initial
bytes was defined to be monotonically decreasing. Thus, when precopy
initial bytes reached zero for all VFIO devices, a single ACK would be
sent and its validity would hold.

However, now the new VFIO_PRECOPY_INFO_REINIT feature allows precopy
initial bytes to be re-initialized during precopy. Specifically, it
means that initial bytes can grow after reaching zero, which would
invalidate a previously sent switchover ACK.

To solve this, make switchover-ack reusable and allow devices to request
switchover ACKs when needed via the save_query_pending SaveVMHandler.

Since now switchover ACK can be requested for a specific device and in
different times, make switchover ACK per-device (instead of a single ACK
for all devices) and let source side do the pending ACKs accounting.

Keep the legacy switchover-ack mechanism for backward compatibility and
turn it on by a compatibility property for older machines. Enable the
property until VFIO implements the new switchover-ack.

Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Link: https://lore.kernel.org/qemu-devel/20260706085211.13905-8-avihaih@nvidia.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>
This commit is contained in:
Avihai Horon
2026-07-06 11:52:02 +03:00
committed by Cédric Le Goater
parent d55f1cf24f
commit de0a1aed07
10 changed files with 124 additions and 38 deletions
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1
include/migration/client-options.h
View File

@@ -13,6 +13,7 @@
/* properties */
bool migrate_send_switchover_start(void);
bool migrate_switchover_ack_legacy(void);
/* capabilities */

2
include/migration/register.h
View File

@@ -23,6 +23,8 @@ typedef struct MigPendingData {
uint64_t postcopy_bytes;
/* Amount of pending bytes can be transferred only in stopcopy */
uint64_t stopcopy_bytes;
/* Number of new pending switchover ACKs */
uint32_t switchover_ack_pending;
/*
* Total pending data, modules do not need to update this field, it
* will be automatically calculated by migration core API.