Re: [Qemu-devel] [PATCH] virtio-net: allow increasing rx queue size

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Aug 04, 2016 at 02:16:14AM +0300, Michael S. Tsirkin wrote:
> This allows increasing the rx queue size up to 1024: unlike with tx,
> guests don't put in huge S/G lists into RX so the risk of running into
> the max 1024 limitation due to some off-by-one seems small.
> 
> It's helpful for users like OVS-DPDK which don't do any buffering on the
> host - 1K roughly matches 500 entries in tun + 256 in the current rx
> queue, which seems to work reasonably well. We could probably make do
> with ~750 entries but virtio spec limits us to powers of two.
> It might be a good idea to specify an s/g size limit in a future
> version.
> 
> It also might be possible to make the queue size smaller down the road, 64
> seems like the minimal value which will still work (as guests seem to
> assume a queue full of 1.5K buffers is enough to process the largest
> incoming packet, which is ~64K).  No one actually asked for this, and
> with virtio 1 guests can reduce ring size without need for host
> configuration, so don't bother with this for now.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Forgot to say: this was originally suggested by Patrik Hermansson
<phermansson@gmail.com>


> ---
>  include/hw/virtio/virtio-net.h |  1 +
>  hw/net/virtio-net.c            | 22 +++++++++++++++++++++-
>  2 files changed, 22 insertions(+), 1 deletion(-)
> 
> diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> index 91ed97c..0ced975 100644
> --- a/include/hw/virtio/virtio-net.h
> +++ b/include/hw/virtio/virtio-net.h
> @@ -35,6 +35,7 @@ typedef struct virtio_net_conf
>      uint32_t txtimer;
>      int32_t txburst;
>      char *tx;
> +    uint16_t rx_queue_size;
>  } virtio_net_conf;
>  
>  /* Maximum packet size we can receive from tap device: header + 64k */
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index 01f1351..4e595e9 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -1412,7 +1412,8 @@ static void virtio_net_add_queue(VirtIONet *n, int index)
>  {
>      VirtIODevice *vdev = VIRTIO_DEVICE(n);
>  
> -    n->vqs[index].rx_vq = virtio_add_queue(vdev, 256, virtio_net_handle_rx);
> +    n->vqs[index].rx_vq = virtio_add_queue(vdev, n->net_conf.rx_queue_size,
> +                                           virtio_net_handle_rx);
>      if (n->net_conf.tx && !strcmp(n->net_conf.tx, "timer")) {
>          n->vqs[index].tx_vq =
>              virtio_add_queue(vdev, 256, virtio_net_handle_tx_timer);
> @@ -1716,10 +1717,28 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
>      VirtIONet *n = VIRTIO_NET(dev);
>      NetClientState *nc;
>      int i;
> +    int min_rx_queue_size;
>  
>      virtio_net_set_config_size(n, n->host_features);
>      virtio_init(vdev, "virtio-net", VIRTIO_ID_NET, n->config_size);
>  
> +    /*
> +     * We set a lower limit on RX queue size to what it always was.
> +     * Guests that want a smaller ring can always resize it without
> +     * help from us (using virtio 1 and up).
> +     */
> +    min_rx_queue_size = 256;
> +    if (n->net_conf.rx_queue_size < min_rx_queue_size ||
> +        n->net_conf.rx_queue_size > VIRTQUEUE_MAX_SIZE ||
> +        (n->net_conf.rx_queue_size & (n->net_conf.rx_queue_size - 1))) {
> +        error_setg(errp, "Invalid rx_queue_size (= %" PRIu16 "), "
> +                   "must be a power of 2 between %d and %d.",
> +                   n->net_conf.rx_queue_size, min_rx_queue_size,
> +                   VIRTQUEUE_MAX_SIZE);
> +        virtio_cleanup(vdev);
> +        return;
> +    }
> +
>      n->max_queues = MAX(n->nic_conf.peers.queues, 1);
>      if (n->max_queues * 2 + 1 > VIRTIO_QUEUE_MAX) {
>          error_setg(errp, "Invalid number of queues (= %" PRIu32 "), "
> @@ -1880,6 +1899,7 @@ static Property virtio_net_properties[] = {
>                         TX_TIMER_INTERVAL),
>      DEFINE_PROP_INT32("x-txburst", VirtIONet, net_conf.txburst, TX_BURST),
>      DEFINE_PROP_STRING("tx", VirtIONet, net_conf.tx),
> +    DEFINE_PROP_UINT16("rx_queue_size", VirtIONet, net_conf.rx_queue_size, 256),
>      DEFINE_PROP_END_OF_LIST(),
>  };
>  
> -- 
> MST