Re: [PATCH 4/5] virtio-net: Update virtio-net curr_queue_pairs in vdpa backends

[Jason Wang <jasowang@redhat.com>]

On Thu, Aug 25, 2022 at 10:53 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Thu, Aug 25, 2022 at 8:38 AM Si-Wei Liu <si-wei.liu@oracle.com> wrote:
> >
> >
> >
> > On 8/23/2022 9:27 PM, Jason Wang wrote:
> > >
> > > 在 2022/8/20 01:13, Eugenio Pérez 写道:
> > >> It was returned as error before. Instead of it, simply update the
> > >> corresponding field so qemu can send it in the migration data.
> > >>
> > >> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> > >> ---
> > >
> > >
> > > Looks correct.
> > >
> > > Adding Si Wei for double check.
> > Hmmm, I understand why this change is needed for live migration, but
> > this would easily cause userspace out of sync with the kernel for other
> > use cases, such as link down or userspace fallback due to vdpa ioctl
> > error. Yes, these are edge cases.
>
> Considering 7.2 will start, maybe it's time to fix the root cause
> instead of having a workaround like this?

Btw, the patch actually tries its best to limit the behaviour, e.g it
doesn't do the following set_status() stuff. So I think it won't
trigger the issue you mentioned here?

Thanks

>
> THanks
>
> > Not completely against it, but I
> > wonder if there's a way we can limit the change scope to live migration
> > case only?
> >
> > -Siwei
> >
> > >
> > > Thanks
> > >
> > >
> > >>   hw/net/virtio-net.c | 17 ++++++-----------
> > >>   1 file changed, 6 insertions(+), 11 deletions(-)
> > >>
> > >> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > >> index dd0d056fde..63a8332cd0 100644
> > >> --- a/hw/net/virtio-net.c
> > >> +++ b/hw/net/virtio-net.c
> > >> @@ -1412,19 +1412,14 @@ static int virtio_net_handle_mq(VirtIONet *n,
> > >> uint8_t cmd,
> > >>           return VIRTIO_NET_ERR;
> > >>       }
> > >>   -    /* Avoid changing the number of queue_pairs for vdpa device in
> > >> -     * userspace handler. A future fix is needed to handle the mq
> > >> -     * change in userspace handler with vhost-vdpa. Let's disable
> > >> -     * the mq handling from userspace for now and only allow get
> > >> -     * done through the kernel. Ripples may be seen when falling
> > >> -     * back to userspace, but without doing it qemu process would
> > >> -     * crash on a recursive entry to virtio_net_set_status().
> > >> -     */
> > >> +    n->curr_queue_pairs = queue_pairs;
> > >>       if (nc->peer && nc->peer->info->type ==
> > >> NET_CLIENT_DRIVER_VHOST_VDPA) {
> > >> -        return VIRTIO_NET_ERR;
> > >> +        /*
> > >> +         * Avoid updating the backend for a vdpa device: We're only
> > >> interested
> > >> +         * in updating the device model queues.
> > >> +         */
> > >> +        return VIRTIO_NET_OK;
> > >>       }
> > >> -
> > >> -    n->curr_queue_pairs = queue_pairs;
> > >>       /* stop the backend before changing the number of queue_pairs
> > >> to avoid handling a
> > >>        * disabled queue */
> > >>       virtio_net_set_status(vdev, vdev->status);
> > >
> >