vfio/migration: Add VFIO migration pre-copy support

Pre-copy support allows the VFIO device data to be transferred while the
VM is running. This helps to accommodate VFIO devices that have a large
amount of data that needs to be transferred, and it can reduce migration
downtime.

Pre-copy support is optional in VFIO migration protocol v2.
Implement pre-copy of VFIO migration protocol v2 and use it for devices
that support it. Full description of it can be found in the following
Linux commit: 4db52602a607 ("vfio: Extend the device migration protocol
with PRE_COPY").

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Tested-by: YangHang Liu <yanghliu@redhat.com>
Acked-by: Alex Williamson <alex.williamson@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>

1 files changed, 25 insertions, 10 deletions

diff --git a/docs/devel/vfio-migration.rst b/docs/devel/vfio-migration.rst
index 1b68ccf115..e896b2a673 100644
--- a/docs/devel/vfio-migration.rst
+++ b/docs/devel/vfio-migration.rst
@@ -7,12 +7,14 @@ 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 currently consists of a single stop-and-copy phase.
-During the stop-and-copy phase the guest is stopped and the entire VFIO device
-data is transferred to the destination.
-
-The pre-copy phase of migration is currently not supported for VFIO devices.
-Support for VFIO pre-copy will be added later on.
+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 opt-in to pre-copy
+support by reporting the VFIO_MIGRATION_PRE_COPY flag in the
+VFIO_DEVICE_FEATURE_MIGRATION ioctl.
 
 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
@@ -29,10 +31,20 @@ VFIO implements the device hooks for the iterative approach as follows:
 * A ``load_setup`` function that sets the VFIO device on the destination in
   _RESUMING state.
 
+* A ``state_pending_estimate`` function that reports an estimate of the
+  remaining pre-copy data that the vendor driver has yet to save for the VFIO
+  device.
+
 * A ``state_pending_exact`` 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.
 
+* An ``is_active_iterate`` function that indicates ``save_live_iterate`` is
+  active only when the VFIO device is in pre-copy states.
+
+* A ``save_live_iterate`` function that reads the VFIO device's data from the
+  vendor driver during iterative pre-copy phase.
+
 * A ``save_state`` function to save the device config space if it is present.
 
 * A ``save_live_complete_precopy`` function that sets the VFIO device in
@@ -111,8 +123,10 @@ 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 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
 ------------------------
@@ -124,11 +138,12 @@ Live migration save path
                                   |
                      migrate_init spawns migration_thread
                 Migration thread then calls each device's .save_setup()
-                       (RUNNING, _SETUP, _RUNNING)
+                  (RUNNING, _SETUP, _RUNNING [_PRE_COPY])
                                   |
-                      (RUNNING, _ACTIVE, _RUNNING)
-             If device is active, get pending_bytes by .state_pending_exact()
+                  (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