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

[Jason Wang]

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?

>
>  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.

> +
>  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.

> +        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?

>      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);
>  }
>