Re: [Qemu-devel] [PATCH] net/virtio: fix multi-queue negotiation

[Marcel Apfelbaum]

On 06/24/2015 11:00 AM, Jason Wang wrote:
> On 06/19/2015 02:05 AM, Marcel Apfelbaum wrote:
> > Clear host multi-queue related features if the peer
> > doesn't support it.
> >
> > Signed-off-by: Marcel Apfelbaum <address@hidden>
> > ---
> > Notes:
> >   This fixes a guest CPU soft lock, however the virtio-net
> >   device will not work correctly. It seems that is
> >   peer's "fault", not knowing how to handle the situation.
> >   However, I submit this patch since it corrects the negotiation
> >   and saves us from a guest crash (!).
>
> Could you please describe how to reproduce this issue?
Hi Jason,
Sorry for the late reply.
I was hoping that a vhost-user multi-queue "insider" will ask questions :)

This happens when we have OVS as backend without multi-queue support
and qemu/guest with multi-queue-support.


> >   Any ideas from the virtio/multi-queue developers on how to debug
> >   this further are welcomed.
> >
> >   hw/net/virtio-net.c | 13 ++++++++++++-
> >   1 file changed, 12 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > index 9281aa1..63e59e8 100644
> > --- a/hw/net/virtio-net.c
> > +++ b/hw/net/virtio-net.c
> > @@ -367,6 +367,11 @@ static int peer_has_ufo(VirtIONet *n)
> >       return n->has_ufo;
> >   }
> >
> > +static int peer_has_multiqueue(VirtIONet *n)
> > +{
> > +    return n->multiqueue;
> > +}
>
> The name is confusing, this is in fact whether or not guest support
> multiqueue. To check peer's ability, you need check
> n->nic_conf.peers.queues instead.
I just wanted to mimic similar code, I have no issue against
this approach.

> > +
> >   static void virtio_net_set_mrg_rx_bufs(VirtIONet *n, int mergeable_rx_bufs,
> >                                          int version_1)
> >   {
> > @@ -469,6 +474,13 @@ static uint64_t virtio_net_get_features(VirtIODevice 
> > *vdev, uint64_t features)
> >           virtio_clear_feature(&features, VIRTIO_NET_F_HOST_UFO);
> >       }
> >
> > +    if (!peer_has_multiqueue(n)) {
> > +        virtio_clear_feature(&features, VIRTIO_NET_F_MQ);
>
> I'm not quite understand this, VIRTIO_NET_F_MQ should work if peer has
> only 1 queue.
Please explain, I thought  VIRTIO_NET_F_MQ flag *enables*
multi-queue, and in our case we pass queue=2 to vhost.

> > +        virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_ANNOUNCE);
> > +        virtio_clear_feature(&features, VIRTIO_NET_F_CTRL_VQ);
> > +        virtio_clear_feature(&features, VIRTIO_NET_F_CTRL_RX);
> > +    }
> > +
>
> Those features don't depend on multiqueue, why clear them?
Once VIRTIO_NET_F_MQ is cleared, the virtio driver from guest is complaining
about those other flags too.

Any explanations about how *it should* work are welcomed.
Again, the scenario is: backend doesn't support multi-queue, QEMU/guest do,
and queues=2 is passed on command line.


Thanks,
Marcel
> >       if (!get_vhost_net(nc->peer)) {
> >           virtio_add_feature(&features, VIRTIO_F_VERSION_1);
> >           return features;
> > @@ -1314,7 +1326,6 @@ static void virtio_net_tx_bh(void *opaque)
> >   static void virtio_net_set_multiqueue(VirtIONet *n, int multiqueue)
> >   {
> >       n->multiqueue = multiqueue;
> > -
> >       virtio_net_set_queues(n);
> >   }