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vfio/migration: Add P2P support for VFIO migration 
VFIO migration uAPI defines an optional 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.

The purpose of this state is to support migration of multiple devices
that might do P2P transactions between themselves.

Add support for P2P migration by transitioning all the devices to the
P2P quiescent state before stopping or starting the devices. Use the new
VMChangeStateHandler prepare_cb to achieve that behavior.

This will allow migration of multiple VFIO devices if all of them
support P2P migration.

Signed-off-by: Avihai Horon <avihaih@nvidia.com>
Tested-by: YangHang Liu <yanghliu@redhat.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
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@avihai1122 @legoater
avihai1122 authored and legoater committed Sep 11, 2023
1 parent 3d4d0f0 commit 94f775e
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93 changes: 57 additions & 36 deletions docs/devel/vfio-migration.rst
View file
Expand Up @@ -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
Expand Down Expand Up @@ -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
========
Expand Down
6 changes: 4 additions & 2 deletions hw/vfio/common.c
View file
Expand Up @@ -441,14 +441,16 @@ bool vfio_device_state_is_running(VFIODevice *vbasedev)
{
VFIOMigration *migration = vbasedev->migration;

return migration->device_state == VFIO_DEVICE_STATE_RUNNING;
return migration->device_state == VFIO_DEVICE_STATE_RUNNING ||
migration->device_state == VFIO_DEVICE_STATE_RUNNING_P2P;
}

bool vfio_device_state_is_precopy(VFIODevice *vbasedev)
{
VFIOMigration *migration = vbasedev->migration;

return migration->device_state == VFIO_DEVICE_STATE_PRE_COPY;
return migration->device_state == VFIO_DEVICE_STATE_PRE_COPY ||
migration->device_state == VFIO_DEVICE_STATE_PRE_COPY_P2P;
}

static bool vfio_devices_all_dirty_tracking(VFIOContainer *container)
Expand Down
46 changes: 43 additions & 3 deletions hw/vfio/migration.c
View file
Expand Up @@ -71,8 +71,12 @@ static const char *mig_state_to_str(enum vfio_device_mig_state state)
return "STOP_COPY";
case VFIO_DEVICE_STATE_RESUMING:
return "RESUMING";
case VFIO_DEVICE_STATE_RUNNING_P2P:
return "RUNNING_P2P";
case VFIO_DEVICE_STATE_PRE_COPY:
return "PRE_COPY";
case VFIO_DEVICE_STATE_PRE_COPY_P2P:
return "PRE_COPY_P2P";
default:
return "UNKNOWN STATE";
}
Expand Down Expand Up @@ -652,6 +656,39 @@ static const SaveVMHandlers savevm_vfio_handlers = {

/* ---------------------------------------------------------------------- */

static void vfio_vmstate_change_prepare(void *opaque, bool running,
RunState state)
{
VFIODevice *vbasedev = opaque;
VFIOMigration *migration = vbasedev->migration;
enum vfio_device_mig_state new_state;
int ret;

new_state = migration->device_state == VFIO_DEVICE_STATE_PRE_COPY ?
VFIO_DEVICE_STATE_PRE_COPY_P2P :
VFIO_DEVICE_STATE_RUNNING_P2P;

/*
* If setting the device in new_state fails, the device should be reset.
* To do so, use ERROR state as a recover state.
*/
ret = vfio_migration_set_state(vbasedev, new_state,
VFIO_DEVICE_STATE_ERROR);
if (ret) {
/*
* Migration should be aborted in this case, but vm_state_notify()
* currently does not support reporting failures.
*/
if (migrate_get_current()->to_dst_file) {
qemu_file_set_error(migrate_get_current()->to_dst_file, ret);
}
}

trace_vfio_vmstate_change_prepare(vbasedev->name, running,
RunState_str(state),
mig_state_to_str(new_state));
}

static void vfio_vmstate_change(void *opaque, bool running, RunState state)
{
VFIODevice *vbasedev = opaque;
Expand Down Expand Up @@ -758,6 +795,7 @@ static int vfio_migration_init(VFIODevice *vbasedev)
char id[256] = "";
g_autofree char *path = NULL, *oid = NULL;
uint64_t mig_flags = 0;
VMChangeStateHandler *prepare_cb;

if (!vbasedev->ops->vfio_get_object) {
return -EINVAL;
Expand Down Expand Up @@ -798,9 +836,11 @@ static int vfio_migration_init(VFIODevice *vbasedev)
register_savevm_live(id, VMSTATE_INSTANCE_ID_ANY, 1, &savevm_vfio_handlers,
vbasedev);

migration->vm_state = qdev_add_vm_change_state_handler(vbasedev->dev,
vfio_vmstate_change,
vbasedev);
prepare_cb = migration->mig_flags & VFIO_MIGRATION_P2P ?
vfio_vmstate_change_prepare :
NULL;
migration->vm_state = qdev_add_vm_change_state_handler_full(
vbasedev->dev, vfio_vmstate_change, prepare_cb, vbasedev);
migration->migration_state.notify = vfio_migration_state_notifier;
add_migration_state_change_notifier(&migration->migration_state);

Expand Down
1 change: 1 addition & 0 deletions hw/vfio/trace-events
View file
Expand Up @@ -167,3 +167,4 @@ vfio_save_setup(const char *name, uint64_t data_buffer_size) " (%s) data buffer
vfio_state_pending_estimate(const char *name, uint64_t precopy, uint64_t postcopy, uint64_t precopy_init_size, uint64_t precopy_dirty_size) " (%s) precopy 0x%"PRIx64" postcopy 0x%"PRIx64" precopy initial size 0x%"PRIx64" precopy dirty size 0x%"PRIx64
vfio_state_pending_exact(const char *name, uint64_t precopy, uint64_t postcopy, uint64_t stopcopy_size, uint64_t precopy_init_size, uint64_t precopy_dirty_size) " (%s) precopy 0x%"PRIx64" postcopy 0x%"PRIx64" stopcopy size 0x%"PRIx64" precopy initial size 0x%"PRIx64" precopy dirty size 0x%"PRIx64
vfio_vmstate_change(const char *name, int running, const char *reason, const char *dev_state) " (%s) running %d reason %s device state %s"
vfio_vmstate_change_prepare(const char *name, int running, const char *reason, const char *dev_state) " (%s) running %d reason %s device state %s"

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