Re: [PATCH v1 2/2] virtio-net: virtio_net_flush_tx() check for per-queue reset

[Xuan Zhuo]

On Sun, 29 Jan 2023 03:12:12 -0500, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> On Sun, Jan 29, 2023 at 03:28:28PM +0800, Xuan Zhuo wrote:
> > On Sun, 29 Jan 2023 02:25:43 -0500, "Michael S. Tsirkin" <mst@redhat.com> 
> > wrote:
> > > On Sun, Jan 29, 2023 at 10:51:50AM +0800, Xuan Zhuo wrote:
> > > > Check whether it is per-queue reset state in virtio_net_flush_tx().
> > > >
> > > > Before per-queue reset, we need to recover async tx resources. At this
> > > > time, virtio_net_flush_tx() is called, but we should not try to send
> > > > new packets, so virtio_net_flush_tx() should check the current
> > > > per-queue reset state.
> > >
> > >
> > > What does "at this time" mean here?
> > > Do you in fact mean it's called from flush_or_purge_queued_packets?
> >
> > Yes
> >
> > virtio_queue_reset
> >     k->queue_reset
> >             virtio_net_queue_reset
> >                     flush_or_purge_queued_packets
> >                             qemu_flush_or_purge_queued_packets
> >                                     .....
> >                                     (callback) virtio_net_tx_complete
> >                                             virtio_net_flush_tx <-- here 
> > send new packet. We need stop it.
> >
> >
> > Because it is inside the callback, I can't pass information through the 
> > stack. I
> > originally thought it was a general situation, so I wanted to put it in
> > struct VirtQueue.
> >
> > If it is not very suitable, it may be better to put it in VirtIONetQueue.
> >
> > Thanks.
>
> Hmm maybe. Another idea: isn't virtio_net_tx_complete called
> with length 0 here? Are there other cases where length is 0?
>
>
> > > What does the call stack look like?
> > >
> > > If yes introducing a vq state just so virtio_net_flush_tx
> > > knows we are in the process of reset would be a bad idea.
> > > We want something much more local, ideally on stack even ...
> > >
> > >
> > > >
> > > > Fixes: 7dc6be52 ("virtio-net: support queue reset")
> > > > Fixes: https://gitlab.com/qemu-project/qemu/-/issues/1451
> > > > Reported-by: Alexander Bulekov <alxndr@bu.edu>
> > > > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > > > ---
> > > >  hw/net/virtio-net.c | 3 ++-
> > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > > index 3ae909041a..fba6451a50 100644
> > > > --- a/hw/net/virtio-net.c
> > > > +++ b/hw/net/virtio-net.c
> > > > @@ -2627,7 +2627,8 @@ static int32_t virtio_net_flush_tx(VirtIONetQueue 
> > > > *q)
> > > >      VirtQueueElement *elem;
> > > >      int32_t num_packets = 0;
> > > >      int queue_index = vq2q(virtio_get_queue_index(q->tx_vq));
> > > > -    if (!(vdev->status & VIRTIO_CONFIG_S_DRIVER_OK)) {
> > > > +    if (!(vdev->status & VIRTIO_CONFIG_S_DRIVER_OK) ||
> > > > +        virtio_queue_reset_state(q->tx_vq)) {
>
> btw this sounds like you are asking it to reset some state.
>
> > > >          return num_packets;
>
> and then
>
>     ret = virtio_net_flush_tx(q);
>     if (ret >= n->tx_burst)
>
>
> will reschedule automatically won't it?
>
> also why check in virtio_net_flush_tx and not virtio_net_tx_complete?

virtio_net_flush_tx may been called by timer.

Thanks.

>
>
> > > >      }
> > > >
> > > > --
> > > > 2.32.0.3.g01195cf9f
> > >
>