Subject: Re: [PATCH net-next 2/3] vhost: introduce vhost_vq_more_avail()

From: Jason Wang <[email-masked]>
Date: Tue, 1 Dec 2015 13:14:10 +0800

On 11/30/2015 04:22 PM, Michael S. Tsirkin wrote:
> On Wed, Nov 25, 2015 at 03:11:28PM +0800, Jason Wang wrote:
>> Signed-off-by: Jason Wang <[email-masked]>
>> ---
>>  drivers/vhost/vhost.c | 26 +++++++++++++++++---------
>>  drivers/vhost/vhost.h |  1 +
>>  2 files changed, 18 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index 163b365..b86c5aa 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -1633,10 +1633,25 @@ 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) {
>> +		vq_err(vq, "Failed to check avail idx at %p: %d\n",
>> +		       &vq->avail->idx, r);
>> +		return false;
> In patch 3 you are calling this under preempt disable,
> so this actually can fail and it isn't a VQ error.
>

Yes.

>> +	}
>> +
>> +	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)
>>  {
>> -	__virtio16 avail_idx;
>>  	int r;
>>  
>>  	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
>> @@ -1660,14 +1675,7 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>>  	/* They could have slipped one in as we were doing that: make
>>  	 * sure it's written, then check again. */
>>  	smp_mb();
>> -	r = __get_user(avail_idx, &vq->avail->idx);
>> -	if (r) {
>> -		vq_err(vq, "Failed to check avail idx at %p: %d\n",
>> -		       &vq->avail->idx, r);
>> -		return false;
>> -	}
>> -
>> -	return vhost16_to_cpu(vq, avail_idx) != vq->avail_idx;
>> +	return vhost_vq_more_avail(dev, vq);
>>  }
>>  EXPORT_SYMBOL_GPL(vhost_enable_notify);
>>  
> This path does need an error though.
> It's probably easier to just leave this call site alone.

Ok, will leave this function as is and remove the vq_err() in
vhost_vq_more_avail().

Thanks

>
>> 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