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Diffstat (limited to 'docs/devel/migration/vfio.rst')
| -rw-r--r-- | docs/devel/migration/vfio.rst | 45 |
1 files changed, 43 insertions, 2 deletions
diff --git a/docs/devel/migration/vfio.rst b/docs/devel/migration/vfio.rst index c49482eab6..673e354754 100644 --- a/docs/devel/migration/vfio.rst +++ b/docs/devel/migration/vfio.rst @@ -67,15 +67,35 @@ VFIO implements the device hooks for the iterative approach as follows: * A ``switchover_ack_needed`` function that checks if the VFIO device uses "switchover-ack" migration capability when this capability is enabled. -* A ``save_state`` function to save the device config space if it is present. +* A ``switchover_start`` function that in the multifd mode starts a thread that + 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 + 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. +* A ``load_state_buffer`` function that loads the device state and the device + config that arrived via multifd channels. + It's used only in the multifd mode. + * ``cleanup`` functions for both save and load that perform any migration related cleanup. @@ -176,8 +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 @@ -194,6 +217,9 @@ Live migration resume path (RESTORE_VM, _ACTIVE, _STOP) | For each device, .load_state() is called for that device section data + transmitted via the main migration channel. + For data transmitted via multifd channels .load_state_buffer() is called + instead. (RESTORE_VM, _ACTIVE, _RESUMING) | At the end, .load_cleanup() is called for each device and vCPUs are started @@ -206,3 +232,18 @@ Postcopy ======== Postcopy migration is currently not supported for VFIO devices. + +Multifd +======= + +Starting from QEMU version 10.0 there's a possibility to transfer VFIO device +_STOP_COPY state via multifd channels. This helps reduce downtime - especially +with multiple VFIO devices or with devices having a large migration state. +As an additional benefit, setting the VFIO device to _STOP_COPY state and +saving its config space is also parallelized (run in a separate thread) in +such migration mode. + +The multifd VFIO device state transfer is controlled by +"x-migration-multifd-transfer" VFIO device property. This property defaults to +AUTO, which means that VFIO device state transfer via multifd channels is +attempted in configurations that otherwise support it. |