Subject: Re: [PATCH V2 2/3] vhost: introduce vhost_vq_more_avail()

From: Jason Wang <[email-masked]>
Date: Fri, 22 Jan 2016 13:43:14 +0800

On 01/20/2016 10:09 PM, Michael S. Tsirkin wrote:
> On Tue, Dec 01, 2015 at 02:39:44PM +0800, Jason Wang wrote:
>> Signed-off-by: Jason Wang <[email-masked]>
> Wow new API with no comments anywhere, and no
> commit log to say what it's good for.
> Want to know what it does and whether
> it's correct? You have to read the next patch.
>
> So what is the point of splitting it out?
> It's confusing, and in fact it made you
> miss a bug.

Ok, will add comments to explain the function.

>
>> ---
>>  drivers/vhost/vhost.c | 13 +++++++++++++
>>  drivers/vhost/vhost.h |  1 +
>>  2 files changed, 14 insertions(+)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 163b365..4f45a03 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -1633,6 +1633,19 @@ void vhost_add_used_and_signal_n(struct vhost_dev *dev,
>>  }
>>  EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);
>>  
>> +bool vhost_vq_more_avail(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>> +{
>> +	__virtio16 avail_idx;
>> +	int r;
>> +
>> +	r = __get_user(avail_idx, &vq->avail->idx);
>> +	if (r)
>> +		return false;
> So the result is that if the page is not present,
> you return false (empty ring) and the
> caller will busy wait with preempt disabled.
> Nasty.
>
> So it should return something that breaks
> the loop, and this means it should have
> a different name for the return code
> to make sense.
>
> Maybe reverse the polarity: vhost_vq_avail_empty?
> And add a comment saying we can't be sure ring
> is empty so return false.

Sounds good, will do this.

>
>> +
>> +	return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx;
>> +}
>> +EXPORT_SYMBOL_GPL(vhost_vq_more_avail);
>> +
>>  /* OK, now we need to know about added descriptors. */
>>  bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>>  {
>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>> index 43284ad..2f3c57c 100644
>> --- a/drivers/vhost/vhost.h
>> +++ b/drivers/vhost/vhost.h
>> @@ -159,6 +159,7 @@ void vhost_add_used_and_signal_n(struct vhost_dev *, struct vhost_virtqueue *,
>>  			       struct vring_used_elem *heads, unsigned count);
>>  void vhost_signal(struct vhost_dev *, struct vhost_virtqueue *);
>>  void vhost_disable_notify(struct vhost_dev *, struct vhost_virtqueue *);
>> +bool vhost_vq_more_avail(struct vhost_dev *, struct vhost_virtqueue *);
>>  bool vhost_enable_notify(struct vhost_dev *, struct vhost_virtqueue *);
>>  
>>  int vhost_log_write(struct vhost_virtqueue *vq, struct vhost_log *log,
>> -- 
>> 2.5.0
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