Re: [RFC PATCH net-next 2/6] virtio: introduce virtio_enable_cb_avail()

[Jason Wang <jasowang@redhat.com>]

On 10/15/2014 05:28 PM, Michael S. Tsirkin wrote:
> On Wed, Oct 15, 2014 at 03:25:26PM +0800, Jason Wang wrote:
>> This patch introduces virtio_enable_cb_avail() to publish avail idx
>> and used event. This could be used by batched buffer submitting to
>> reduce the number of tx interrupts.
>>
>> Cc: Rusty Russell <rusty@rustcorp.com.au>
>> Cc: Michael S. Tsirkin <mst@redhat.com>
>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>> ---
>>  drivers/virtio/virtio_ring.c |   22 ++++++++++++++++++++--
>>  include/linux/virtio.h       |    2 ++
>>  2 files changed, 22 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> index 1b3929f..d67fbf8 100644
>> --- a/drivers/virtio/virtio_ring.c
>> +++ b/drivers/virtio/virtio_ring.c
>> @@ -567,14 +567,32 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
>>  	 * entry. Always do both to keep code simple. */
>>  	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
>>  	/* Make sure used event never go backwards */
>> -	if (!vring_need_event(vring_used_event(&vq->vring),
>> -			      vq->vring.avail->idx, last_used_idx))
>> +	if (vq->vring.avail->idx != vring_used_event(&vq->vring) &&
>> +	    !vring_need_event(vring_used_event(&vq->vring),
>> +			      vq->vring.avail->idx, last_used_idx)) {
>>  		vring_used_event(&vq->vring) = last_used_idx;
>> +	}
>>  	END_USE(vq);
>>  	return last_used_idx;
>>  }
>>  EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
>>
> I see you are also changing virtqueue_enable_cb_prepare, why?

This is also used to prevent it from moving the used event backwards.
This may happens when we handle tx napi after we publish avail idx as
used event (virtqueue_enable_cb_avail() was called).
>
>> +bool virtqueue_enable_cb_avail(struct virtqueue *_vq)
>> +{
>> +	struct vring_virtqueue *vq = to_vvq(_vq);
>> +	bool ret;
>> +
>> +	START_USE(vq);
>> +	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
>> +	vring_used_event(&vq->vring) = vq->vring.avail->idx;
>> +	ret = vring_need_event(vq->vring.avail->idx,
>> +			       vq->last_used_idx, vq->vring.used->idx);
>> +	END_USE(vq);
>> +
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(virtqueue_enable_cb_avail);
>> +
>>  /**
>>   * virtqueue_poll - query pending used buffers
>>   * @vq: the struct virtqueue we're talking about.
> Could not figure out what this does.
> Please add documentation.
>

Sure, does something like below explain what does this function do?

/**                                                                            

 * virtqueue_enable_cb_avail - restart callbacks after
disable_cb.           
 * @vq: the struct virtqueue we're talking
about.                              
 *                                                                             

 * This re-enables callbacks but hints to the other side to
delay              
 * interrupts all of the available buffers have been processed;         
 * it returns "false" if there are at least one pending buffer in the
queue,          
 * to detect a possible race between the driver checking for more
work,        
 * and enabling
callbacks.                                                     
 *                                                                             

 * Caller must ensure we don't call this with other
virtqueue                  
 * operations at the same time (except where
noted).                           
 */

>> diff --git a/include/linux/virtio.h b/include/linux/virtio.h
>> index b46671e..bfaf058 100644
>> --- a/include/linux/virtio.h
>> +++ b/include/linux/virtio.h
>> @@ -65,6 +65,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
>>  
>>  unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
>>  
>> +bool virtqueue_enable_cb_avail(struct virtqueue *vq);
>> +
>>  bool virtqueue_poll(struct virtqueue *vq, unsigned);
>>  
>>  bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
>> -- 
>> 1.7.1
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