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

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

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?

> Thanks,
> Tiwei
> 
>>
>> I'm not strictly against this change but I'm wondering if it's worth
>> it.
>>
>> regards,
>> Jens
>>