On 2/4/2026 3:24 PM, Markus Armbruster wrote:
External email: Use caution opening links or attachments
Markus Armbruster <[email protected]> writes:
Avihai Horon <[email protected]> writes:
The VFIO_MIGRATION event notifies users when a VFIO device transitions
to a new state.
One use case for this event is to prevent timeouts for RDMA connections
to the migrated device. In this case, an external management application
(not libvirt) consumes the events and disables the RDMA timeout
mechanism when receiving the event for PRE_COPY_P2P state, which
indicates that the device is non-responsive.
This is essential because RDMA connections typically have very low
timeouts (tens of milliseconds), which can be far below migration
downtime.
However, under heavy resource utilization, the device transition to
PRE_COPY_P2P can take hundreds of milliseconds to complete. Since the
VFIO_MIGRATION event is currently sent only after the transition
completes, it arrives too late, after RDMA connections have already
timed out.
To address this, send an additional "prepare" event immediately before
initiating the PRE_COPY_P2P transition. This guarantees timely event
delivery regardless of how long the actual state transition takes.
Signed-off-by: Avihai Horon <[email protected]>
[...]
diff --git a/qapi/vfio.json b/qapi/vfio.json
index a1a9c5b673..17b6046871 100644
--- a/qapi/vfio.json
+++ b/qapi/vfio.json
@@ -11,7 +11,13 @@
##
# @QapiVfioMigrationState:
#
-# An enumeration of the VFIO device migration states.
+# An enumeration of the VFIO device migration states. In addition to
+# the regular states, there are prepare states (with 'prepare' suffix)
+# which indicate that the device is just about to transition to the
+# corresponding state. Note that seeing a prepare state for state X
+# doesn't guarantee that the next state will be X, as the state
+# transition can fail and the device may transition to a different
+# state instead.
#
# @stop: The device is stopped.
#
@@ -32,11 +38,14 @@
# tracking its internal state and its internal state is available
# for reading.
#
+# @pre-copy-p2p-prepare: The device is just about to move to
+# pre-copy-p2p state. (since 11.0)
+#
# Since: 9.1
##
{ 'enum': 'QapiVfioMigrationState',
'data': [ 'stop', 'running', 'stop-copy', 'resuming', 'running-p2p',
- 'pre-copy', 'pre-copy-p2p' ] }
+ 'pre-copy', 'pre-copy-p2p', 'pre-copy-p2p-prepare' ] }
##
# @VFIO_MIGRATION:
Acked-by: Markus Armbruster <[email protected]>
Except for the subject line: "vfio/migration: Send VFIO_MIGRATION event
before PRE_COPY_P2P transition" become misleading in v2.
Can you explain why misleading?
Prior to this patch VFIO_MIGRATION event was sent only after
PRE_COPY_P2P transition.
Now with this patch VFIO_MIGRATION event is sent also before
PRE_COPY_P2P transition.
Thanks.
