Re: [PATCH] virtio-net: set the max of queue size to 4096 according to the virtio specs, The maximum Queue Size value is 32768 : https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-240006

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

subject is unreasonably long.
limit it to 50 characters. esp links do not belong there.

Looks like this is a repost, ignoring most comments
on the previous version.

This kind of thing is ok as a conversation starter but then
pls add RFC in the subject.


On Mon, Sep 19, 2022 at 08:28:22AM -0400, liuhaiwei wrote:
> the limit of maximum of rx_queue_size and tx_queue to 1024 is so small as to affect our network performance when using the  virtio-net and vhost ,
> we cannot set the maximum size beyond 1k.


don't use such long lines in the commit log.  The line length for the
body text of a commit message should be constrained to 80 characters. 

> why not enlarge the maximum size (such as 4096) when using the vhost backend?

because we might then theoretically get a single s/g exceeding 1k,
and that does not fit in an iovec.

> 
> Signed-off-by: liuhaiwei <liuhaiwei9699@126.com>
> ---
>  hw/net/virtio-net.c        | 47 +++++++++++++++++++++++++++-----------
>  hw/virtio/virtio.c         |  8 +++++--
>  include/hw/virtio/virtio.h |  1 +
>  3 files changed, 41 insertions(+), 15 deletions(-)
> 
> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> index dd0d056fde..4b56484855 100644
> --- a/hw/net/virtio-net.c
> +++ b/hw/net/virtio-net.c
> @@ -52,12 +52,11 @@
>  #define MAX_VLAN    (1 << 12)   /* Per 802.1Q definition */
>  
>  /* previously fixed value */
> -#define VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE 256
> -#define VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE 256
> +#define VIRTIO_NET_VHOST_USER_DEFAULT_SIZE 2048
>  
>  /* for now, only allow larger queue_pairs; with virtio-1, guest can downsize */
> -#define VIRTIO_NET_RX_QUEUE_MIN_SIZE VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE
> -#define VIRTIO_NET_TX_QUEUE_MIN_SIZE VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE
> +#define VIRTIO_NET_RX_QUEUE_MIN_SIZE 256
> +#define VIRTIO_NET_TX_QUEUE_MIN_SIZE 256
>  
>  #define VIRTIO_NET_IP4_ADDR_SIZE   8        /* ipv4 saddr + daddr */
>  
> @@ -594,6 +593,28 @@ static int peer_has_ufo(VirtIONet *n)
>      return n->has_ufo;
>  }
>  
> +static void virtio_net_set_default_queue_size(VirtIONet *n)
> +{
> +    NetClientState *peer = n->nic_conf.peers.ncs[0];
> +
> +    /* Default value is 0 if not set */
> +    if (n->net_conf.rx_queue_size == 0) {
> +        if (peer && peer->info->type == NET_CLIENT_DRIVER_VHOST_USER) {
> +            n->net_conf.rx_queue_size = VIRTIO_NET_VHOST_USER_DEFAULT_SIZE;
> +        } else {
> +            n->net_conf.rx_queue_size = VIRTIO_NET_VQ_MAX_SIZE;
> +        }
> +    }
> +
> +    if (n->net_conf.tx_queue_size == 0) {
> +        if (peer && peer->info->type == NET_CLIENT_DRIVER_VHOST_USER) {
> +            n->net_conf.tx_queue_size = VIRTIO_NET_VHOST_USER_DEFAULT_SIZE;
> +        } else {
> +            n->net_conf.tx_queue_size = VIRTIO_NET_VQ_MAX_SIZE;
> +        }
> +    }
> +}
> +
>  static void virtio_net_set_mrg_rx_bufs(VirtIONet *n, int mergeable_rx_bufs,
>                                         int version_1, int hash_report)
>  {
> @@ -633,7 +654,7 @@ static int virtio_net_max_tx_queue_size(VirtIONet *n)
>       * size.
>       */
>      if (!peer) {
> -        return VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;
> +        return VIRTIO_NET_VQ_MAX_SIZE;
>      }
>  
>      switch(peer->info->type) {
> @@ -641,7 +662,7 @@ static int virtio_net_max_tx_queue_size(VirtIONet *n)
>      case NET_CLIENT_DRIVER_VHOST_VDPA:
>          return VIRTQUEUE_MAX_SIZE;
>      default:
> -        return VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE;
> +        return VIRTIO_NET_VQ_MAX_SIZE;
>      };
>  }
>  
> @@ -3450,30 +3471,30 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
>  
>      virtio_net_set_config_size(n, n->host_features);
>      virtio_init(vdev, VIRTIO_ID_NET, n->config_size);
> -
> +    virtio_net_set_default_queue_size(n);
>      /*
>       * 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).
>       */
>      if (n->net_conf.rx_queue_size < VIRTIO_NET_RX_QUEUE_MIN_SIZE ||
> -        n->net_conf.rx_queue_size > VIRTQUEUE_MAX_SIZE ||
> +        n->net_conf.rx_queue_size > VIRTIO_NET_VQ_MAX_SIZE ||
>          !is_power_of_2(n->net_conf.rx_queue_size)) {
>          error_setg(errp, "Invalid rx_queue_size (= %" PRIu16 "), "
>                     "must be a power of 2 between %d and %d.",
>                     n->net_conf.rx_queue_size, VIRTIO_NET_RX_QUEUE_MIN_SIZE,
> -                   VIRTQUEUE_MAX_SIZE);
> +                   VIRTIO_NET_VQ_MAX_SIZE );
>          virtio_cleanup(vdev);
>          return;
>      }
>  
>      if (n->net_conf.tx_queue_size < VIRTIO_NET_TX_QUEUE_MIN_SIZE ||
> -        n->net_conf.tx_queue_size > VIRTQUEUE_MAX_SIZE ||
> +        n->net_conf.tx_queue_size > VIRTIO_NET_VQ_MAX_SIZE ||
>          !is_power_of_2(n->net_conf.tx_queue_size)) {
>          error_setg(errp, "Invalid tx_queue_size (= %" PRIu16 "), "
>                     "must be a power of 2 between %d and %d",
>                     n->net_conf.tx_queue_size, VIRTIO_NET_TX_QUEUE_MIN_SIZE,
> -                   VIRTQUEUE_MAX_SIZE);
> +                   VIRTIO_NET_VQ_MAX_SIZE);
>          virtio_cleanup(vdev);
>          return;
>      }
> @@ -3751,9 +3772,9 @@ static Property virtio_net_properties[] = {
>      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,
> -                       VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE),
> +                       0),
>      DEFINE_PROP_UINT16("tx_queue_size", VirtIONet, net_conf.tx_queue_size,
> -                       VIRTIO_NET_TX_QUEUE_DEFAULT_SIZE),
> +                       0),
>      DEFINE_PROP_UINT16("host_mtu", VirtIONet, net_conf.mtu, 0),
>      DEFINE_PROP_BOOL("x-mtu-bypass-backend", VirtIONet, mtu_bypass_backend,
>                       true),
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 5d607aeaa0..ad9dfa20e7 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -2286,11 +2286,15 @@ void virtio_queue_set_rings(VirtIODevice *vdev, int n, hwaddr desc,
>  
>  void virtio_queue_set_num(VirtIODevice *vdev, int n, int num)
>  {
> +    int vq_max_size = VIRTQUEUE_MAX_SIZE;
> +    if (!strcmp(vdev->name, "virtio-net")) {
> +        vq_max_size = VIRTIO_NET_VQ_MAX_SIZE;
> +    }
>      /* Don't allow guest to flip queue between existent and
>       * nonexistent states, or to set it to an invalid size.
>       */
>      if (!!num != !!vdev->vq[n].vring.num ||
> -        num > VIRTQUEUE_MAX_SIZE ||
> +        num > vq_max_size ||
>          num < 0) {
>          return;
>      }
> @@ -2423,7 +2427,7 @@ VirtQueue *virtio_add_queue(VirtIODevice *vdev, int queue_size,
>              break;
>      }
>  
> -    if (i == VIRTIO_QUEUE_MAX || queue_size > VIRTQUEUE_MAX_SIZE)
> +    if (i == VIRTIO_QUEUE_MAX )
>          abort();
>  
>      vdev->vq[i].vring.num = queue_size;
> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> index db1c0ddf6b..1f4d2eb5d7 100644
> --- a/include/hw/virtio/virtio.h
> +++ b/include/hw/virtio/virtio.h
> @@ -50,6 +50,7 @@ size_t virtio_feature_get_config_size(const VirtIOFeature *features,
>  typedef struct VirtQueue VirtQueue;
>  
>  #define VIRTQUEUE_MAX_SIZE 1024
> +#define VIRTIO_NET_VQ_MAX_SIZE (4096)

() not needed

>  typedef struct VirtQueueElement
>  {
> -- 
> 2.27.0