[dpdk-dev] [PATCH 4/5] net/vhost: remove limit of vhost TX burst size

[Kevin Traynor]

On 02/24/2017 08:48 AM, Zhiyong Yang wrote:
> vhost removes limit of TX burst size(32 pkts) and supports to make
> an best effort to transmit pkts.
> 
> Cc: yuanhan.liu at linux.intel.com
> Cc: maxime.coquelin at redhat.com
> 
> Signed-off-by: Zhiyong Yang <zhiyong.yang at intel.com>
> ---
>  drivers/net/vhost/rte_eth_vhost.c | 24 ++++++++++++++++++++++--
>  1 file changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/vhost/rte_eth_vhost.c b/drivers/net/vhost/rte_eth_vhost.c
> index e98cffd..1e1fa34 100644
> --- a/drivers/net/vhost/rte_eth_vhost.c
> +++ b/drivers/net/vhost/rte_eth_vhost.c
> @@ -52,6 +52,7 @@
>  #define ETH_VHOST_QUEUES_ARG		"queues"
>  #define ETH_VHOST_CLIENT_ARG		"client"
>  #define ETH_VHOST_DEQUEUE_ZERO_COPY	"dequeue-zero-copy"
> +#define VHOST_MAX_PKT_BURST 32
>  
>  static const char *valid_arguments[] = {
>  	ETH_VHOST_IFACE_ARG,
> @@ -434,8 +435,27 @@ eth_vhost_tx(void *q, struct rte_mbuf **bufs, uint16_t nb_bufs)
>  		goto out;
>  
>  	/* Enqueue packets to guest RX queue */
> -	nb_tx = rte_vhost_enqueue_burst(r->vid,
> -			r->virtqueue_id, bufs, nb_bufs);
> +	if (likely(nb_bufs <= VHOST_MAX_PKT_BURST))
> +		nb_tx = rte_vhost_enqueue_burst(r->vid, r->virtqueue_id,
> +						bufs, nb_bufs);
> +	else {
> +		uint16_t nb_send = nb_bufs;
> +
> +		while (nb_send) {
> +			uint16_t nb_pkts;
> +			uint16_t num = (uint16_t)RTE_MIN(nb_send,
> +					VHOST_MAX_PKT_BURST);
> +
> +			nb_pkts = rte_vhost_enqueue_burst(r->vid,
> +							  r->virtqueue_id,
> +							  &bufs[nb_tx], num);
> +
> +			nb_tx += nb_pkts;
> +			nb_send -= nb_pkts;
> +			if (nb_pkts < num)
> +				break;
> +		}

In the code above,
- if the VM does not service the queue, this will spin forever
- if the queue is almost full, it will be very slow

In the example of OVS, other ports being serviced by the same core would
stop being serviced or drop a lot of packets.

If you want to remove the 32 pkt limitation, the
rte_vhost_enqueue_burst() api limitation needs to be changed first. It
doesn't make sense to put retry code in eth_vhost_tx() fn, the
application can retry if it wants to.

> +	}
>  
>  	r->stats.pkts += nb_tx;
>  	r->stats.missed_pkts += nb_bufs - nb_tx;
>