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

[Eugenio Perez Martin]

On Thu, Aug 25, 2022 at 2:38 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.

The link down case is not possible at this moment because that cvq
command does not call virtio_net_handle_ctrl_iov. A similar treatment
than mq would be needed when supported, and the call to
virtio_net_set_status will be avoided.

I'll double check device initialization ioctl failure with
n->curr_queue_pairs > 1 in the destination, but I think we should be
safe.

> Not completely against it, but I
> wonder if there's a way we can limit the change scope to live migration
> case only?
>

The reason to update the device model is to send the curr_queue_pairs
to the destination in a backend agnostic way. To send it otherwise
would limit the live migration possibilities, but sure we can explore
another way.

Thanks!