forked from mirror/qemu
You cannot select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
151 lines
6.8 KiB
ReStructuredText
151 lines
6.8 KiB
ReStructuredText
=====================
|
|
VFIO device Migration
|
|
=====================
|
|
|
|
Migration of virtual machine involves saving the state for each device that
|
|
the guest is running on source host and restoring this saved state on the
|
|
destination host. This document details how saving and restoring of VFIO
|
|
devices is done in QEMU.
|
|
|
|
Migration of VFIO devices consists of two phases: the optional pre-copy phase,
|
|
and the stop-and-copy phase. The pre-copy phase is iterative and allows to
|
|
accommodate VFIO devices that have a large amount of data that needs to be
|
|
transferred. The iterative pre-copy phase of migration allows for the guest to
|
|
continue whilst the VFIO device state is transferred to the destination, this
|
|
helps to reduce the total downtime of the VM. VFIO devices can choose to skip
|
|
the pre-copy phase of migration by returning pending_bytes as zero during the
|
|
pre-copy phase.
|
|
|
|
A detailed description of the UAPI for VFIO device migration can be found in
|
|
the comment for the ``vfio_device_migration_info`` structure in the header
|
|
file linux-headers/linux/vfio.h.
|
|
|
|
VFIO implements the device hooks for the iterative approach as follows:
|
|
|
|
* A ``save_setup`` function that sets up the migration region and sets _SAVING
|
|
flag in the VFIO device state.
|
|
|
|
* A ``load_setup`` function that sets up the migration region on the
|
|
destination and sets _RESUMING flag in the VFIO device state.
|
|
|
|
* A ``save_live_pending`` function that reads pending_bytes from the vendor
|
|
driver, which indicates the amount of data that the vendor driver has yet to
|
|
save for the VFIO device.
|
|
|
|
* A ``save_live_iterate`` function that reads the VFIO device's data from the
|
|
vendor driver through the migration region during iterative phase.
|
|
|
|
* A ``save_state`` function to save the device config space if it is present.
|
|
|
|
* A ``save_live_complete_precopy`` function that resets _RUNNING flag from the
|
|
VFIO device state and iteratively copies the remaining data for the VFIO
|
|
device until the vendor driver indicates that no data remains (pending bytes
|
|
is zero).
|
|
|
|
* A ``load_state`` function that loads the config section and the data
|
|
sections that are generated by the save functions above
|
|
|
|
* ``cleanup`` functions for both save and load that perform any migration
|
|
related cleanup, including unmapping the migration region
|
|
|
|
|
|
The VFIO migration code uses a VM state change handler to change the VFIO
|
|
device state when the VM state changes from running to not-running, and
|
|
vice versa.
|
|
|
|
Similarly, a migration state change handler is used to trigger a transition of
|
|
the VFIO device state when certain changes of the migration state occur. For
|
|
example, the VFIO device state is transitioned back to _RUNNING in case a
|
|
migration failed or was canceled.
|
|
|
|
System memory dirty pages tracking
|
|
----------------------------------
|
|
|
|
A ``log_global_start`` and ``log_global_stop`` memory listener callback informs
|
|
the VFIO IOMMU module to start and stop dirty page tracking. A ``log_sync``
|
|
memory listener callback marks those system memory pages as dirty which are
|
|
used for DMA by the VFIO device. The dirty pages bitmap is queried per
|
|
container. All pages pinned by the vendor driver through external APIs have to
|
|
be marked as dirty during migration. When there are CPU writes, CPU dirty page
|
|
tracking can identify dirtied pages, but any page pinned by the vendor driver
|
|
can also be written by the device. There is currently no device or IOMMU
|
|
support for dirty page tracking in hardware.
|
|
|
|
By default, dirty pages are tracked when the device is in pre-copy as well as
|
|
stop-and-copy phase. So, a page pinned by the vendor driver will be copied to
|
|
the destination in both phases. Copying dirty pages in pre-copy phase helps
|
|
QEMU to predict if it can achieve its downtime tolerances. If QEMU during
|
|
pre-copy phase keeps finding dirty pages continuously, then it understands
|
|
that even in stop-and-copy phase, it is likely to find dirty pages and can
|
|
predict the downtime accordingly.
|
|
|
|
QEMU also provides a per device opt-out option ``pre-copy-dirty-page-tracking``
|
|
which disables querying the dirty bitmap during pre-copy phase. If it is set to
|
|
off, all dirty pages will be copied to the destination in stop-and-copy phase
|
|
only.
|
|
|
|
System memory dirty pages tracking when vIOMMU is enabled
|
|
---------------------------------------------------------
|
|
|
|
With vIOMMU, an IO virtual address range can get unmapped while in pre-copy
|
|
phase of migration. In that case, the unmap ioctl returns any dirty pages in
|
|
that range and QEMU reports corresponding guest physical pages dirty. During
|
|
stop-and-copy phase, an IOMMU notifier is used to get a callback for mapped
|
|
pages and then dirty pages bitmap is fetched from VFIO IOMMU modules for those
|
|
mapped ranges.
|
|
|
|
Flow of state changes during Live migration
|
|
===========================================
|
|
|
|
Below is the flow of state change during live migration.
|
|
The values in the brackets represent the VM state, the migration state, and
|
|
the VFIO device state, respectively.
|
|
|
|
Live migration save path
|
|
------------------------
|
|
|
|
::
|
|
|
|
QEMU normal running state
|
|
(RUNNING, _NONE, _RUNNING)
|
|
|
|
|
migrate_init spawns migration_thread
|
|
Migration thread then calls each device's .save_setup()
|
|
(RUNNING, _SETUP, _RUNNING|_SAVING)
|
|
|
|
|
(RUNNING, _ACTIVE, _RUNNING|_SAVING)
|
|
If device is active, get pending_bytes by .save_live_pending()
|
|
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 _SAVING state
|
|
(FINISH_MIGRATE, _DEVICE, _SAVING)
|
|
|
|
|
For the VFIO device, iterate in .save_live_complete_precopy until
|
|
pending data is 0
|
|
(FINISH_MIGRATE, _DEVICE, _STOPPED)
|
|
|
|
|
(FINISH_MIGRATE, _COMPLETED, _STOPPED)
|
|
Migraton thread schedules cleanup bottom half and exits
|
|
|
|
Live migration resume path
|
|
--------------------------
|
|
|
|
::
|
|
|
|
Incoming migration calls .load_setup for each device
|
|
(RESTORE_VM, _ACTIVE, _STOPPED)
|
|
|
|
|
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)
|
|
|
|
Postcopy
|
|
========
|
|
|
|
Postcopy migration is currently not supported for VFIO devices.
|