Re: [PATCH 18/31] vhost: Shadow virtqueue buffers forwarding

[Eugenio Perez Martin <eperezma@redhat.com>]

On Mon, Feb 21, 2022 at 8:44 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> 在 2022/2/17 下午8:48, Eugenio Perez Martin 写道:
> > On Tue, Feb 8, 2022 at 9:16 AM Jason Wang <jasowang@redhat.com> wrote:
> >>
> >> 在 2022/2/1 下午7:25, Eugenio Perez Martin 写道:
> >>> On Sun, Jan 30, 2022 at 7:47 AM Jason Wang <jasowang@redhat.com> wrote:
> >>>> 在 2022/1/22 上午4:27, Eugenio Pérez 写道:
> >>>>> @@ -272,6 +590,28 @@ void vhost_svq_set_svq_kick_fd(VhostShadowVirtqueue *svq, int svq_kick_fd)
> >>>>>     void vhost_svq_stop(VhostShadowVirtqueue *svq)
> >>>>>     {
> >>>>>         event_notifier_set_handler(&svq->svq_kick, NULL);
> >>>>> +    g_autofree VirtQueueElement *next_avail_elem = NULL;
> >>>>> +
> >>>>> +    if (!svq->vq) {
> >>>>> +        return;
> >>>>> +    }
> >>>>> +
> >>>>> +    /* Send all pending used descriptors to guest */
> >>>>> +    vhost_svq_flush(svq, false);
> >>>> Do we need to wait for all the pending descriptors to be completed here?
> >>>>
> >>> No, this function does not wait, it only completes the forwarding of
> >>> the *used* descriptors.
> >>>
> >>> The best example is the net rx queue in my opinion. This call will
> >>> check SVQ's vring used_idx and will forward the last used descriptors
> >>> if any, but all available descriptors will remain as available for
> >>> qemu's VQ code.
> >>>
> >>> To skip it would miss those last rx descriptors in migration.
> >>>
> >>> Thanks!
> >>
> >> So it's probably to not the best place to ask. It's more about the
> >> inflight descriptors so it should be TX instead of RX.
> >>
> >> I can imagine the migration last phase, we should stop the vhost-vDPA
> >> before calling vhost_svq_stop(). Then we should be fine regardless of
> >> inflight descriptors.
> >>
> > I think I'm still missing something here.
> >
> > To be on the same page. Regarding tx this could cause repeated tx
> > frames (one at source and other at destination), but never a missed
> > buffer not transmitted. The "stop before" could be interpreted as "SVQ
> > is not forwarding available buffers anymore". Would that work?
>
>
> Right, but this only work if
>
> 1) a flush to make sure TX DMA for inflight descriptors are all completed
>
> 2) just mark all inflight descriptor used
>

It currently trusts on the reverse: Buffers not marked as used (by the
device) will be available in the destination, so expect
retransmissions.

Thanks!

> Otherwise there could be buffers that is inflight forever.
>
> Thanks
>
>
> >
> > Thanks!
> >
> >> Thanks
> >>
> >>
> >>>> Thanks
> >>>>
> >>>>
> >>>>> +
> >>>>> +    for (unsigned i = 0; i < svq->vring.num; ++i) {
> >>>>> +        g_autofree VirtQueueElement *elem = NULL;
> >>>>> +        elem = g_steal_pointer(&svq->ring_id_maps[i]);
> >>>>> +        if (elem) {
> >>>>> +            virtqueue_detach_element(svq->vq, elem, elem->len);
> >>>>> +        }
> >>>>> +    }
> >>>>> +
> >>>>> +    next_avail_elem = g_steal_pointer(&svq->next_guest_avail_elem);
> >>>>> +    if (next_avail_elem) {
> >>>>> +        virtqueue_detach_element(svq->vq, next_avail_elem,
> >>>>> +                                 next_avail_elem->len);
> >>>>> +    }
> >>>>>     }
>