[PATCH v3 1/1] docs/devel: Add VFIO device migration documentation

[Tarun Gupta]

Document interfaces used for VFIO device migration. Added flow of state changes
during live migration with VFIO device. Tested by building docs with the new
vfio-migration.rst file.

v3:
- Add introductory line about VM migration in general.
- Remove occurcences of vfio_pin_pages() to describe pinning.
- Incorporated comments from v2

v2:
- Included the new vfio-migration.rst file in index.rst
- Updated dirty page tracking section, also added details about
  'pre-copy-dirty-page-tracking' opt-out option.
- Incorporated comments around wording of doc.

Signed-off-by: Tarun Gupta <targupta@nvidia.com>
Signed-off-by: Kirti Wankhede <kwankhede@nvidia.com>
---
 MAINTAINERS                   |   1 +
 docs/devel/index.rst          |   1 +
 docs/devel/vfio-migration.rst | 143 ++++++++++++++++++++++++++++++++++
 3 files changed, 145 insertions(+)
 create mode 100644 docs/devel/vfio-migration.rst

diff --git a/MAINTAINERS b/MAINTAINERS
index 738786146d..a2a80eee59 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -1801,6 +1801,7 @@ M: Alex Williamson <alex.williamson@redhat.com>
 S: Supported
 F: hw/vfio/*
 F: include/hw/vfio/
+F: docs/devel/vfio-migration.rst
 
 vfio-ccw
 M: Cornelia Huck <cohuck@redhat.com>
diff --git a/docs/devel/index.rst b/docs/devel/index.rst
index ae664da00c..5330f1ca1d 100644
--- a/docs/devel/index.rst
+++ b/docs/devel/index.rst
@@ -39,3 +39,4 @@ Contents:
    qom
    block-coroutine-wrapper
    multi-process
+   vfio-migration
diff --git a/docs/devel/vfio-migration.rst b/docs/devel/vfio-migration.rst
new file mode 100644
index 0000000000..24cb55991a
--- /dev/null
+++ b/docs/devel/vfio-migration.rst
@@ -0,0 +1,143 @@
+=====================
+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 device hooks for iterative approach:
+
+* A ``save_setup`` function that sets up the migration region, sets _SAVING
+  flag in the VFIO device state and informs the VFIO IOMMU module to start
+  dirty page tracking.
+
+* 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_live_complete_precopy`` function that resets _RUNNING flag from the
+  VFIO device state, saves the device config space, if any, 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
+
+A VM state change handler is registered to change the VFIO device state when
+the VM state changes.
+
+Similarly, a migration state change notifier is registered to get a
+notification on migration state change. These states are translated to the
+corresponding VFIO device state and conveyed to the vendor driver.
+
+System memory dirty pages 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 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 vendor driver will be copied to
+destination in both the 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 per device opt-out option ``pre-copy-dirty-page-tracking``
+which disables querying dirty bitmap during pre-copy phase. If it is set to
+off, all dirty pages will be copied to 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.
-- 
2.27.0