Re: [RFC PATCH for 8.0 00/13] vDPA-net inflight descriptors migration with SVQ

[Eugenio Perez Martin]

On Tue, Dec 6, 2022 at 8:08 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Tue, Dec 6, 2022 at 1:04 AM Eugenio Pérez <eperezma@redhat.com> wrote:
> >
> > The state of the descriptors (avail or used) may not be recoverable just
> > looking at the guest memory.  Out of order used descriptor may override
> > previous avail ones in the descriptor table or avail vring.
> >
> > Currently we're not migrating this status in net devices because virtio-net,
> > vhost-kernel etc use the descriptors in order,
>
> Note that this might not be the truth (when zerocopy is enabled).
>

Good point. So will virtio-net wait for those to complete then? How
does qemu handle if there are still inflight descriptors?

> > so the information always
> > recoverable from guest's memory.  However, vDPA devices may use them out of
> > order, and other kind of devices like block need this support.
> >
> > Shadow virtqueue is able to track these and resend them at the destination.
>
> As discussed, there's a bootstrap issue here:
>
> When SVQ needs to be enabled on demand, do we still need another way
> to get inflight ones without the help of SVQ?
>

To send and retrieve the descriptor without SVQ needs to be developed
on top of this. I should have made that more clear here in the cover
letter.

Thanks!

> Thanks
>
> > Add them to the virtio-net migration description so they are not lose in the
> > process.
> >
> > This is a very early RFC just to validate the first draft so expect 
> > leftovers.
> > To fetch and request the descriptors from a device without SVQ need to be
> > implemented on top. Some other notable pending items are:
> > * Do not send the descriptors actually recoverable from the guest memory.
> > * Properly version the migrate data.
> > * Properly abstract the descriptors access from virtio-net to SVQ.
> > * Do not use VirtQueueElementOld but migrate directly VirtQueueElement.
> > * Replace lots of assertions with runtime conditionals.
> > * Other TODOs in the patch message or code changes.
> >
> > Thanks.
> >
> > Eugenio Pérez (13):
> >   vhost: add available descriptor list in SVQ
> >   vhost: iterate only available descriptors at SVQ stop
> >   vhost: merge avail list and next avail descriptors detach
> >   vhost: add vhost_svq_save_inflight
> >   virtio: Specify uint32_t as VirtQueueElementOld members type
> >   virtio: refactor qemu_get_virtqueue_element
> >   virtio: refactor qemu_put_virtqueue_element
> >   virtio: expose VirtQueueElementOld
> >   virtio: add vmstate_virtqueue_element_old
> >   virtio-net: Migrate vhost inflight descriptors
> >   virtio-net: save inflight descriptors at vhost shutdown
> >   vhost: expose vhost_svq_add_element
> >   vdpa: Recover inflight descriptors
> >
> >  hw/virtio/vhost-shadow-virtqueue.h |   9 ++
> >  include/hw/virtio/virtio-net.h     |   2 +
> >  include/hw/virtio/virtio.h         |  32 ++++++
> >  include/migration/vmstate.h        |  22 ++++
> >  hw/net/vhost_net.c                 |  56 ++++++++++
> >  hw/net/virtio-net.c                | 129 +++++++++++++++++++++++
> >  hw/virtio/vhost-shadow-virtqueue.c |  52 +++++++--
> >  hw/virtio/vhost-vdpa.c             |  11 --
> >  hw/virtio/virtio.c                 | 162 ++++++++++++++++++-----------
> >  9 files changed, 392 insertions(+), 83 deletions(-)
> >
> > --
> > 2.31.1
> >
> >
>