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

[Jens Freimann <jfreimann@redhat.com>]

On Wed, Sep 12, 2018 at 10:25:57AM +0200, Maxime Coquelin wrote:
>
>
>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?

Safer but also less explicit for someone who just reads the higher
level function. But I think it's better to go for the safer version
here.  

>
>>+}
>>+
>>+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;

yes, I will either change to what you suggested or
get rid of the helper completely since it is only used in
very few places.

Thanks for the review!

regards,
Jens