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

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

On 3/19/19 2:47 PM, Jens Freimann wrote:
> 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.

OK, when I meant later, I meant to remove vq_ prefix for all fields, not
only vq_split & vq_packed.

But yes, that's just cosmetic so let's keep it as is for now.

> 
> regards,
> Jens