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

[Tiwei Bie <tiwei.bie@intel.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?

Sounds good to me.