Re: [dpdk-dev] [PATCH v5 03/11] net/virtio: add packed virtqueue helpers

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 09/06/2018 08:19 PM, Jens Freimann wrote:
> Add helper functions to set/clear and check descriptor flags.
> 
> Signed-off-by: Jens Freimann <jfreimann@redhat.com>
> ---
>   drivers/net/virtio/virtio_ring.h | 26 ++++++++++++++++++++++++++
>   drivers/net/virtio/virtqueue.h   | 19 +++++++++++++++++++
>   2 files changed, 45 insertions(+)
> 
> diff --git a/drivers/net/virtio/virtio_ring.h b/drivers/net/virtio/virtio_ring.h
> index e2c597434..f3b23f419 100644
> --- a/drivers/net/virtio/virtio_ring.h
> +++ b/drivers/net/virtio/virtio_ring.h
> @@ -78,6 +78,8 @@ struct vring_packed_desc_event {
>   
>   struct vring {
>   	unsigned int num;
> +	unsigned int avail_wrap_counter;
> +	unsigned int used_wrap_counter;
>   	union {
>   		struct vring_desc_packed *desc_packed;
>   		struct vring_desc *desc;
> @@ -92,6 +94,30 @@ struct vring {
>   	};
>   };
>   
> +static inline void
> +_set_desc_avail(struct vring_desc_packed *desc, int wrap_counter)
> +{
> +	desc->flags |= VRING_DESC_F_AVAIL(wrap_counter) |
> +		       VRING_DESC_F_USED(!wrap_counter);

It implies the avail and used bits to be cleared beforehand.
Maybe it would be safer to clear them in the helper?

> +}
> +
> +static inline void
> +set_desc_avail(struct vring *vr, struct vring_desc_packed *desc)
> +{
> +	_set_desc_avail(desc, vr->avail_wrap_counter);
> +}
> +
> +static inline int
> +desc_is_used(struct vring_desc_packed *desc, struct vring *vr)
> +{
> +	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;
> +}
> +
>   /* The standard layout for the ring is a continuous chunk of memory which
>    * looks like this.  We assume num is a power of 2.
>    *
> diff --git a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h
> index d2a0b651a..53fce61b4 100644
> --- a/drivers/net/virtio/virtqueue.h
> +++ b/drivers/net/virtio/virtqueue.h
> @@ -245,6 +245,25 @@ struct virtio_tx_region {
>   			   __attribute__((__aligned__(16)));
>   };
>   
> +static inline uint16_t
> +increment_pq_index(uint16_t idx, size_t ring_size)
> +{
> +	return ++idx >= ring_size ? 0 : idx;
> +}

Not sure this helper is really useful, as it ends up
doing two checks in a row.

> +
> +static inline uint16_t
> +update_pq_avail_index(struct virtqueue *vq)
> +{
> +	uint16_t idx;
> +
> +	idx = increment_pq_index(vq->vq_avail_idx, vq->vq_nentries);
> +	if (idx == 0)
> +		vq->vq_ring.avail_wrap_counter ^= 1;

> +	vq->vq_avail_idx = idx;
> +
> +	return vq->vq_avail_idx;
> +}

So it could be simplified to:

{
	if (++vq->vq_avail_idx >= vq->vq_entries) {
		vq->vq_avail_idx = 0;
		vq->vq_ring.avail_wrap_counter ^= 1;
	}

	return vq->vq_avail_idx;
}

> +
>   static inline void
>   vring_desc_init_packed(struct vring *vr, int n)
>   {
>