Re: [dpdk-dev] [PATCH v8 5/8] net/virtio: implement transmit path for packed queues

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

On Tue, Oct 16, 2018 at 11:01:31AM +0200, Jens Freimann wrote:
[...]
> +void
> +vq_ring_free_chain_packed(struct virtqueue *vq, uint16_t desc_idx)
> +{
> +	struct vring_desc_packed *dp;
> +	struct vq_desc_extra *dxp = NULL, *dxp_tail = NULL;
> +	uint16_t desc_idx_last = desc_idx;
> +
> +	dp  = &vq->vq_ring.desc_packed[desc_idx];
> +	dxp = &vq->vq_descx[desc_idx];

In the out-of-order case, we can't assume the desc and desc_extra
have the same index.

> +	vq->vq_free_cnt = (uint16_t)(vq->vq_free_cnt + dxp->ndescs);
> +	if ((dp->flags & VRING_DESC_F_INDIRECT) == 0) {
> +		while (dp->flags & VRING_DESC_F_NEXT) {

https://github.com/oasis-tcs/virtio-spec/blob/89dd55f5e606/packed-ring.tex#L222-L226
The device only writes out a single used descriptor for the whole
list. It then skips forward according to the number of
descriptors in the list. The driver needs to keep track of the size
of the list corresponding to each buffer ID, to be able to skip
to where the next used descriptor is written by the device.

IOW, we can't use the VRING_DESC_F_NEXT here.


> +			desc_idx_last = dxp->next;
> +			dp = &vq->vq_ring.desc_packed[dxp->next];
> +			dxp = &vq->vq_descx[dxp->next];
> +		}
> +	}