On Tue, Jul 22, 2025 at 8:41 PM Jonah Palmer <jonah.pal...@oracle.com> wrote: > > 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).
It would be better to explain which kind of operations were heavy and time-consuming and how iterative migration help. > > 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. I wonder about the plan for libvirt support. > > 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: Are you testing this via a software virtio device or hardware one? > > 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(-) > > -- > 2.47.1 Thanks >
