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

[Jens Freimann <jfreimann@redhat.com>]

On Tue, Mar 19, 2019 at 02:28:30PM +0100, Maxime Coquelin wrote:
>
>
>On 3/19/19 11:09 AM, Tiwei Bie wrote:
>>On Tue, Mar 19, 2019 at 10:44:32AM +0100, Jens Freimann wrote:
>>>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.
>>
>>I think it will make it clear that fields like used_wrap_counter
>>are only available in packed ring which will make the code more
>>readable.
>>
>>>
>>>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 prefer to have consistent prefix like most fields in this
>>structure (although some fields don't really follow this).
>
>As Jens, I tend to agree that the vq_ prefix is quite redundant.
>However, I think it is better to keep it in this patch for consistency.
>
>Maybe it can be remove in a separate patch later?

I thought it might be convenient to change it now as we are touching
all related code anyway. But I also don't want to block the patch because of
this cosmetic thing. So let's defer it to a later patch set.

regards,
Jens