Re: [dpdk-dev] [PATCH 05/10] net/virtio: refactor virtqueue structure

[Jens Freimann <jfreimann@redhat.com>]

On Tue, Mar 19, 2019 at 02:43:07PM +0800, Tiwei Bie wrote:
>Put split ring and packed ring specific fields into separate
>sub-structures, and also union them as they won't be available
>at the same time.
>
>Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
>---
> drivers/net/virtio/virtio_ethdev.c           | 71 +++++++++---------
> drivers/net/virtio/virtio_rxtx.c             | 66 ++++++++---------
> drivers/net/virtio/virtio_rxtx_simple.h      |  2 +-
> drivers/net/virtio/virtio_rxtx_simple_neon.c |  2 +-
> drivers/net/virtio/virtio_rxtx_simple_sse.c  |  2 +-
> drivers/net/virtio/virtqueue.c               |  6 +-
> drivers/net/virtio/virtqueue.h               | 77 +++++++++++---------
> 7 files changed, 117 insertions(+), 109 deletions(-)
>
[snip]
...
>diff --git a/drivers/net/virtio/virtqueue.h b/drivers/net/virtio/virtqueue.h
>index 80c0c43c3..48b3912e6 100644
>--- a/drivers/net/virtio/virtqueue.h
>+++ b/drivers/net/virtio/virtqueue.h
>@@ -191,17 +191,22 @@ struct vq_desc_extra {
>
> struct virtqueue {
> 	struct virtio_hw  *hw; /**< virtio_hw structure pointer. */
>-	struct vring vq_ring;  /**< vring keeping desc, used and avail */
>-	struct vring_packed ring_packed;  /**< vring keeping descs */
>-	bool used_wrap_counter;
>-	uint16_t cached_flags; /**< cached flags for descs */
>-	uint16_t event_flags_shadow;
>+	union {
>+		struct {
>+			/**< vring keeping desc, used and avail */
>+			struct vring ring;
>+		} vq_split;
>
>-	/**
>-	 * Last consumed descriptor in the used table,
>-	 * trails vq_ring.used->idx.
>-	 */
>-	uint16_t vq_used_cons_idx;
>+		struct {
>+			/**< vring keeping descs and events */
>+			struct vring_packed ring;
>+			bool used_wrap_counter;
>+			uint16_t cached_flags; /**< cached flags for descs */
>+			uint16_t event_flags_shadow;
>+		} vq_packed;
>+	};
>+
>+	uint16_t vq_used_cons_idx; /**< last consumed descriptor */
> 	uint16_t vq_nentries;  /**< vring desc numbers */
> 	uint16_t vq_free_cnt;  /**< num of desc available */
> 	uint16_t vq_avail_idx; /**< sync until needed */

Honest question: What do we really gain by putting it in a union? We
save a little memory. But we also make code less readable IMHO. 

If we do this, can we at least shorten some variable names, like drop
the vq_ prefix? (It's used everywhere like vq->vq_packed*, so with
vq->packed* we don't loose any context).

I'm not strictly against this change but I'm wondering if it's worth
it.  

regards,
Jens