Re: [PATCH v6 06/11] net/virtio: implement transmit path for packed queues

[Tiwei Bie <tiwei.bie@intel.com>]

On Fri, Sep 21, 2018 at 12:33:03PM +0200, Jens Freimann wrote:
[...]
>  
>  static inline int
> -desc_is_used(struct vring_desc_packed *desc, struct vring *vr)
> +_desc_is_used(struct vring_desc_packed *desc)
>  {
>  	uint16_t used, avail;
>  
>  	used = !!(desc->flags & VRING_DESC_F_USED(1));
>  	avail = !!(desc->flags & VRING_DESC_F_AVAIL(1));
>  
> -	return used == avail && used == vr->used_wrap_counter;
> +	return used == avail;
> +
> +}
> +
> +static inline int
> +desc_is_used(struct vring_desc_packed *desc, struct vring *vr)
> +{
> +	uint16_t used;
> +
> +	used = !!(desc->flags & VRING_DESC_F_USED(1));
> +
> +	return _desc_is_used(desc) && used == vr->used_wrap_counter;
>  }
>  
>  /* The standard layout for the ring is a continuous chunk of memory which
> diff --git a/drivers/net/virtio/virtio_rxtx.c b/drivers/net/virtio/virtio_rxtx.c
> index eb891433e..ea6300563 100644
> --- a/drivers/net/virtio/virtio_rxtx.c
> +++ b/drivers/net/virtio/virtio_rxtx.c
> @@ -38,6 +38,7 @@
>  #define  VIRTIO_DUMP_PACKET(m, len) do { } while (0)
>  #endif
>  
> +
>  int
>  virtio_dev_rx_queue_done(void *rxq, uint16_t offset)
>  {
> @@ -165,6 +166,31 @@ virtqueue_dequeue_rx_inorder(struct virtqueue *vq,
>  #endif
>  
>  /* Cleanup from completed transmits. */
> +static void
> +virtio_xmit_cleanup_packed(struct virtqueue *vq)
> +{
> +	uint16_t idx;
> +	uint16_t size = vq->vq_nentries;
> +	struct vring_desc_packed *desc = vq->vq_ring.desc_packed;
> +	struct vq_desc_extra *dxp;
> +
> +	idx = vq->vq_used_cons_idx;
> +	while (_desc_is_used(&desc[idx]) &&

We can't just compare the AVAIL bit and USED bit to
check whether a desc is used.

> +	       vq->vq_free_cnt < size) {
> +		dxp = &vq->vq_descx[idx];

The code is still assuming the descs will be written
back by device in order. The vq->vq_descx[] needs to
be managed e.g. as a list to support the out-of-order
processing. IOW, we can't assume vq->vq_descx[idx]
is corresponding to desc[idx] when device may write
back the descs out of order.

> +		vq->vq_free_cnt += dxp->ndescs;
> +		idx += dxp->ndescs;
> +		idx = idx >= size ? idx - size : idx;
> +		if (idx == 0) {
> +			vq->vq_ring.used_wrap_counter ^= 1;
> +		}
> +		if (dxp->cookie != NULL) {
> +			rte_pktmbuf_free(dxp->cookie);
> +			dxp->cookie = NULL;
> +		}
> +	}
> +}
[...]