This series is an RFC initial implementation of iterative live
migration for virtio-net devices.
The main motivation behind implementing iterative migration for
virtio-net devices is to start on heavy, time-consuming operations
for the destination while the source is still active (i.e. before
the stop-and-copy phase).
The motivation behind this RFC series specifically is to provide an
initial framework for such an implementation and get feedback on the
design and direction.
-------
This implementation of iterative live migration for a virtio-net device
is enabled via setting the migration capability 'virtio-iterative' to
on for both the source & destination, e.g. (HMP):
(qemu) migrate_set_capability virtio-iterative on
The virtio-net device's SaveVMHandlers hooks are registered/unregistered
during the device's realize/unrealize phase.
Currently, this series only sends and loads the vmstate at the start of
migration. The vmstate is still sent (again) during the stop-and-copy
phase, as it is today, to handle any deltas in the state since it was
initially sent. A future patch in this series could avoid having to
re-send and re-load the entire state again and instead focus only on the
deltas.
There is a slight, modest improvement in guest-visible downtime from
this series. More specifically, when using iterative live migration with
a virtio-net device, the downtime contributed by migrating a virtio-net
device decreased from ~3.2ms to ~1.4ms on average:
Before:
-------
vmstate_downtime_load type=non-iterable idstr=0000:00:03.0/virtio-net
instance_id=0 downtime=3594
After:
------
vmstate_downtime_load type=non-iterable idstr=0000:00:03.0/virtio-net
instance_id=0 downtime=1607
This slight improvement is likely due to the initial vmstate_load_state
call "warming up" pages in memory such that, when it's called a second
time during the stop-and-copy phase, allocation and page-fault latencies
are reduced.
-------
Comments, suggestions, etc. are welcome here.
Jonah Palmer (6):
migration: Add virtio-iterative capability
virtio-net: Reorder vmstate_virtio_net and helpers
virtio-net: Add SaveVMHandlers for iterative migration
virtio-net: iter live migration - migrate vmstate
virtio,virtio-net: skip consistency check in virtio_load for iterative
migration
virtio-net: skip vhost_started assertion during iterative migration
hw/net/virtio-net.c | 246 +++++++++++++++++++++++++++------
hw/virtio/virtio.c | 32 +++--
include/hw/virtio/virtio-net.h | 8 ++
include/hw/virtio/virtio.h | 7 +
migration/savevm.c | 1 +
qapi/migration.json | 7 +-
6 files changed, 247 insertions(+), 54 deletions(-)
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2.47.1
