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Diffstat (limited to 'docs')
| -rw-r--r-- | docs/devel/vfio-migration.rst | 93 |
1 files changed, 57 insertions, 36 deletions
diff --git a/docs/devel/vfio-migration.rst b/docs/devel/vfio-migration.rst index b433cb5..605fe60 100644 --- a/docs/devel/vfio-migration.rst +++ b/docs/devel/vfio-migration.rst @@ -23,9 +23,21 @@ and recommends that the initial bytes are sent and loaded in the destination before stopping the source VM. Enabling this migration capability will guarantee that and thus, can potentially reduce downtime even further. -Note that currently VFIO migration is supported only for a single device. This -is due to VFIO migration's lack of P2P support. However, P2P support is planned -to be added later on. +To support migration of multiple devices that might do P2P transactions between +themselves, VFIO migration uAPI defines an intermediate P2P quiescent state. +While in the P2P quiescent state, P2P DMA transactions cannot be initiated by +the device, but the device can respond to incoming ones. Additionally, all +outstanding P2P transactions are guaranteed to have been completed by the time +the device enters this state. + +All the devices that support P2P migration are first transitioned to the P2P +quiescent state and only then are they stopped or started. This makes migration +safe P2P-wise, since starting and stopping the devices is not done atomically +for all the devices together. + +Thus, multiple VFIO devices migration is allowed only if all the devices +support P2P migration. Single VFIO device migration is allowed regardless of +P2P migration support. A detailed description of the UAPI for VFIO device migration can be found in the comment for the ``vfio_device_mig_state`` structure in the header file @@ -132,54 +144,63 @@ will be blocked. Flow of state changes during Live migration =========================================== -Below is the flow of state change during live migration. +Below is the state change flow during live migration for a VFIO device that +supports both precopy and P2P migration. The flow for devices that don't +support it is similar, except that the relevant states for precopy and P2P are +skipped. The values in the parentheses represent the VM state, the migration state, and the VFIO device state, respectively. -The text in the square brackets represents the flow if the VFIO device supports -pre-copy. Live migration save path ------------------------ :: - QEMU normal running state - (RUNNING, _NONE, _RUNNING) - | + QEMU normal running state + (RUNNING, _NONE, _RUNNING) + | migrate_init spawns migration_thread - Migration thread then calls each device's .save_setup() - (RUNNING, _SETUP, _RUNNING [_PRE_COPY]) - | - (RUNNING, _ACTIVE, _RUNNING [_PRE_COPY]) - If device is active, get pending_bytes by .state_pending_{estimate,exact}() - If total pending_bytes >= threshold_size, call .save_live_iterate() - [Data of VFIO device for pre-copy phase is copied] - Iterate till total pending bytes converge and are less than threshold - | - On migration completion, vCPU stops and calls .save_live_complete_precopy for - each active device. The VFIO device is then transitioned into _STOP_COPY state - (FINISH_MIGRATE, _DEVICE, _STOP_COPY) - | - For the VFIO device, iterate in .save_live_complete_precopy until - pending data is 0 - (FINISH_MIGRATE, _DEVICE, _STOP) - | - (FINISH_MIGRATE, _COMPLETED, _STOP) - Migraton thread schedules cleanup bottom half and exits + Migration thread then calls each device's .save_setup() + (RUNNING, _SETUP, _PRE_COPY) + | + (RUNNING, _ACTIVE, _PRE_COPY) + If device is active, get pending_bytes by .state_pending_{estimate,exact}() + If total pending_bytes >= threshold_size, call .save_live_iterate() + Data of VFIO device for pre-copy phase is copied + Iterate till total pending bytes converge and are less than threshold + | + On migration completion, the vCPUs and the VFIO device are stopped + The VFIO device is first put in P2P quiescent state + (FINISH_MIGRATE, _ACTIVE, _PRE_COPY_P2P) + | + 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 + pending data is 0 + | + (POSTMIGRATE, _COMPLETED, _STOP_COPY) + Migraton thread schedules cleanup bottom half and exits + | + .save_cleanup() is called + (POSTMIGRATE, _COMPLETED, _STOP) Live migration resume path -------------------------- :: - Incoming migration calls .load_setup for each device - (RESTORE_VM, _ACTIVE, _STOP) - | - For each device, .load_state is called for that device section data - (RESTORE_VM, _ACTIVE, _RESUMING) - | - At the end, .load_cleanup is called for each device and vCPUs are started - (RUNNING, _NONE, _RUNNING) + Incoming migration calls .load_setup() for each device + (RESTORE_VM, _ACTIVE, _STOP) + | + For each device, .load_state() is called for that device section data + (RESTORE_VM, _ACTIVE, _RESUMING) + | + At the end, .load_cleanup() is called for each device and vCPUs are started + The VFIO device is first put in P2P quiescent state + (RUNNING, _ACTIVE, _RUNNING_P2P) + | + (RUNNING, _NONE, _RUNNING) Postcopy ======== |