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

[Xuan Zhuo]

On Mon, 30 Jan 2023 11:01:40 +0800, Jason Wang <jasowang@redhat.com> wrote:
> On Sun, Jan 29, 2023 at 3:44 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > On Sun, 29 Jan 2023 14:23:21 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > > On Sun, Jan 29, 2023 at 10:52 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> 
> > > 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.
> > > >
> > > > 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)) {
> > >
> > > We have other places that check DRIVER_OK do we need to check queue
> > > reset as well?
> >
> > I checked it again. I still think that the location of other checking 
> > DRIVER_OK
> > does not need to check the queue reset.
>
> For example, if we don't disable can_receive() when the queue is
> reset, it means rx may go for virtio_net_receive_rcu(). It means the
> Qemu is still trying to process the traffic from the network backend
> like tap which may waste cpu cycles.
>
> I think the correct way is to return false when the queue is reset in
> can_receive(), then the backend poll will be disabled (e.g TAP). When
> the queue is enabled again, qemu_flush_queued_packets() will wake up
> the backend polling.
>
> Having had time to check other places but it would be better to
> mention why it doesn't need a check in the changelog.


static bool virtio_net_can_receive(NetClientState *nc)
{
    VirtIONet *n = qemu_get_nic_opaque(nc);
    VirtIODevice *vdev = VIRTIO_DEVICE(n);
    VirtIONetQueue *q = virtio_net_get_subqueue(nc);

    if (!vdev->vm_running) {
        return false;
    }

    if (nc->queue_index >= n->curr_queue_pairs) {
        return false;
    }

    if (!virtio_queue_ready(q->rx_vq) ||
        !(vdev->status & VIRTIO_CONFIG_S_DRIVER_OK)) {
        return false;
    }

    return true;
}

int virtio_queue_ready(VirtQueue *vq)
{
    return vq->vring.avail != 0;
}


static void __virtio_queue_reset(VirtIODevice *vdev, uint32_t i)
{
    vdev->vq[i].vring.desc = 0;
    vdev->vq[i].vring.avail = 0;
    vdev->vq[i].vring.used = 0;
    vdev->vq[i].last_avail_idx = 0;
    vdev->vq[i].shadow_avail_idx = 0;
    vdev->vq[i].used_idx = 0;
    vdev->vq[i].last_avail_wrap_counter = true;
    vdev->vq[i].shadow_avail_wrap_counter = true;
    vdev->vq[i].used_wrap_counter = true;
    virtio_queue_set_vector(vdev, i, VIRTIO_NO_VECTOR);
    vdev->vq[i].signalled_used = 0;
    vdev->vq[i].signalled_used_valid = false;
    vdev->vq[i].notification = true;
    vdev->vq[i].vring.num = vdev->vq[i].vring.num_default;
    vdev->vq[i].inuse = 0;
    virtio_virtqueue_reset_region_cache(&vdev->vq[i]);
}

In the implementation of Per-Queue Reset, for RX, we stop RX by setting 
vdev->vq[i].vring.avail to 0.
Then callback can_receive will return False.


Thanks.


>
> Thanks
>
> >
> > Thanks.
> >
> >
> > >
> > > E.g:
> > > virtio_net_can_receive()
> > > virtio_net_tx_{timer|bh}()
> > >
> > > Thanks
> > >
> > > >          return num_packets;
> > > >      }
> > > >
> > > > --
> > > > 2.32.0.3.g01195cf9f
> > > >
> > >
> >
>