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

[Cornelia Huck]

On Thu, 4 Aug 2016 02:16:14 +0300
"Michael S. Tsirkin" <address@hidden> 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.

Do we need some kind of sanity check that the guest did not resize
below a reasonable limit?

> 
> Signed-off-by: Michael S. Tsirkin <address@hidden>
> ---
>  include/hw/virtio/virtio-net.h |  1 +
>  hw/net/virtio-net.c            | 22 +++++++++++++++++++++-
>  2 files changed, 22 insertions(+), 1 deletion(-)
> 


> @@ -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;

I'd find it more readable to introduce a #define with the old queue
size as the minimum size...

> +    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),

...and defaulting to that #define (or one derived from the #define
above) here.

>      DEFINE_PROP_END_OF_LIST(),
>  };
> 

Do we need compat handling for the new property?