vfio/migration: Multifd device state transfer support - send side

Implement the multifd device state transfer via additional per-device
thread inside save_live_complete_precopy_thread handler.

Switch between doing the data transfer in the new handler and doing it
in the old save_state handler depending if VFIO multifd transfer is enabled
or not.

Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/4d727e2e0435e0022d50004e474077632830e08d.1741124640.git.maciej.szmigiero@oracle.com
[ clg: - Reordered savevm_vfio_handlers
       - Updated save_live_complete_precopy* documentation ]
Signed-off-by: Cédric Le Goater <clg@redhat.com>

1 files changed, 17 insertions, 2 deletions

diff --git a/docs/devel/migration/vfio.rst b/docs/devel/migration/vfio.rst
index d6cf60890c..a803a09bc1 100644
--- a/docs/devel/migration/vfio.rst
+++ b/docs/devel/migration/vfio.rst
@@ -71,11 +71,23 @@ VFIO implements the device hooks for the iterative approach as follows:
   reassembles the multifd received data and loads it in-order into the device.
   In the non-multifd mode this function is a NOP.
 
-* A ``save_state`` function to save the device config space if it is present.
+* A ``save_state`` function to save the device config space if it is present
+  in the non-multifd mode.
+  In the multifd mode it just emits either a dummy EOS marker.
 
 * A ``save_live_complete_precopy`` function that sets the VFIO device in
   _STOP_COPY state and iteratively copies the data for the VFIO device until
   the vendor driver indicates that no data remains.
+  In the multifd mode it just emits a dummy EOS marker.
+
+* A ``save_live_complete_precopy_thread`` function that in the multifd mode
+  provides thread handler performing multifd device state transfer.
+  It sets the VFIO device to _STOP_COPY state, iteratively reads the data
+  from the VFIO device and queues it for multifd transmission until the vendor
+  driver indicates that no data remains.
+  After that, it saves the device config space and queues it for multifd
+  transfer too.
+  In the non-multifd mode this thread is a NOP.
 
 * A ``load_state`` function that loads the config section and the data
   sections that are generated by the save functions above.
@@ -184,8 +196,11 @@ Live migration save path
                 Then the VFIO device is put in _STOP_COPY state
                      (FINISH_MIGRATE, _ACTIVE, _STOP_COPY)
          .save_live_complete_precopy() is called for each active device
-      For the VFIO device, iterate in .save_live_complete_precopy() until
+              For the VFIO device: in the non-multifd mode iterate in
+                        .save_live_complete_precopy() until
                                pending data is 0
+	          In the multifd mode this iteration is done in
+	          .save_live_complete_precopy_thread() instead.
                                       |
                      (POSTMIGRATE, _COMPLETED, _STOP_COPY)
             Migraton thread schedules cleanup bottom half and exits