Re: [Qemu-devel] [PATCH] virtio-net: Make virtio queue sizes configurable

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

On Fri, Nov 27, 2015 at 04:02:39PM +0100, Patrik Hermansson wrote:
> This patch adds the option to specify virtio queue sizes. Currently the
> queue sizes is hard coded to 256, which might not be suitable for all types
> of applications.

Any more data here? 256 packets seems plenty. Is there a chance
your application does not support indirect descriptors?
That's a better approach I think.

> This patch makes it possible to specify the queue size between
> 256 to 1024.
> 
> The minimum value is chosen based on the current sizes of the virtio
> queues. The maximum queue size is based upon VIRTQUEUE_SIZE_MAX.

To me, this seems excessive.  For example, 1024 will break if we have to
add a virtio net header entry (on old kernels).

> ---
>  hw/net/virtio-net.c            | 31 ++++++++++++++++++++++++++++---
>  include/hw/virtio/virtio-net.h |  8 +++++++-
>  2 files changed, 35 insertions(+), 4 deletions(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index a877614..c4fcb39 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -1326,20 +1326,40 @@ static void virtio_net_tx_bh(void *opaque)
>      }
>  }
>  
> +static void virtio_net_validate_queue_limit(uint16_t *queue_size)
> +{
> +    if (*queue_size > VIRTIO_QUEUE_SIZE_MAX) {
> +        error_report("queue-size: %d, exceeds maximum allowed queue-size(%d),"
> +                     "queue-size set to %d", *queue_size, VIRTIO_QUEUE_SIZE_MAX,
> +                      VIRTIO_QUEUE_SIZE_MAX);
> +        *queue_size = VIRTIO_QUEUE_SIZE_MAX;
> +    } else if (*queue_size < VIRTIO_QUEUE_SIZE_MIN) {
> +        error_report("queue-size: %d, below minimum allowed queue-size(%d),"
> +                     "queue-size set to %d", *queue_size, VIRTIO_QUEUE_SIZE_MIN,
> +                      VIRTIO_QUEUE_SIZE_MIN);
> +        *queue_size = VIRTIO_QUEUE_SIZE_MIN;
> +    }
> +}
> +
>  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);
> +    virtio_net_validate_queue_limit(&n->net_conf.rx_virtqueue_sz);
> +    virtio_net_validate_queue_limit(&n->net_conf.tx_virtqueue_sz);
> +    n->vqs[index].rx_vq = virtio_add_queue(vdev, n->net_conf.rx_virtqueue_sz,
> +                                           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);
> +            virtio_add_queue(vdev, n->net_conf.tx_virtqueue_sz,
> +                             virtio_net_handle_tx_timer);
>          n->vqs[index].tx_timer = timer_new_ns(QEMU_CLOCK_VIRTUAL,
>                                                virtio_net_tx_timer,
>                                                &n->vqs[index]);
>      } else {
>          n->vqs[index].tx_vq =
> -            virtio_add_queue(vdev, 256, virtio_net_handle_tx_bh);
> +            virtio_add_queue(vdev, n->net_conf.tx_virtqueue_sz,
> +                             virtio_net_handle_tx_bh);
>          n->vqs[index].tx_bh = qemu_bh_new(virtio_net_tx_bh, &n->vqs[index]);
>      }
>  
> @@ -1826,6 +1846,11 @@ static Property virtio_net_properties[] = {
>      DEFINE_PROP_UINT32("x-txtimer", VirtIONet, net_conf.txtimer,
>                         TX_TIMER_INTERVAL),
>      DEFINE_PROP_INT32("x-txburst", VirtIONet, net_conf.txburst, TX_BURST),
> +    DEFINE_PROP_UINT16("rx_virtqueue_sz", VirtIONet, net_conf.rx_virtqueue_sz,
> +                       RX_VIRTQUEUE_SIZE),
> +    DEFINE_PROP_UINT16("tx_virtqueue_sz", VirtIONet, net_conf.tx_virtqueue_sz,
> +                       TX_VIRTQUEUE_SIZE),
> +
>      DEFINE_PROP_STRING("tx", VirtIONet, net_conf.tx),
>      DEFINE_PROP_END_OF_LIST(),
>  };
> diff --git a/include/hw/virtio/virtio-net.h b/include/hw/virtio/virtio-net.h
> index f3cc25f..dbcabbf 100644
> --- a/include/hw/virtio/virtio-net.h
> +++ b/include/hw/virtio/virtio-net.h
> @@ -28,13 +28,19 @@
>   * ensures fairness in the io path.  256 conveniently matches the
>   * length of the TX queue and shows a good balance of performance
>   * and latency. */
> -#define TX_BURST 256
> +#define VIRTIO_QUEUE_SIZE_MIN 256
> +#define VIRTIO_QUEUE_SIZE_MAX VIRTQUEUE_MAX_SIZE
> +#define RX_VIRTQUEUE_SIZE VIRTIO_QUEUE_SIZE_MIN
> +#define TX_VIRTQUEUE_SIZE VIRTIO_QUEUE_SIZE_MIN
> +#define TX_BURST TX_VIRTQUEUE_SIZE
>  
>  typedef struct virtio_net_conf
>  {
>      uint32_t txtimer;
>      int32_t txburst;
>      char *tx;
> +    uint16_t rx_virtqueue_sz;
> +    uint16_t tx_virtqueue_sz;
>  } virtio_net_conf;
>  
>  /* Maximum packet size we can receive from tap device: header + 64k */
> -- 
> 1.9.1