[Devel] [PATCH vz7.39] scsi: virtio: Reduce BUG if total_sg > virtqueue size to WARN.

[Denis V. Lunev]

On 12/19/2017 05:26 PM, Denis V. Lunev wrote:
> On 12/19/2017 04:02 PM, Roman Kagan wrote:
>> From: "Richard W.M. Jones" <rjones at redhat.com>
>>
>> If using indirect descriptors, you can make the total_sg as large as you
>> want.  If not, BUG is too serious because the function later returns
>> -ENOSPC.
>>
>> Signed-off-by: Richard W.M. Jones <rjones at redhat.com>
>> Reviewed-by: Paolo Bonzini <pbonzini at redhat.com>
>> Signed-off-by: Martin K. Petersen <martin.petersen at oracle.com>
>> (cherry picked from commit 44ed8089e991a60d614abe0ee4b9057a28b364e4)
>> Signed-off-by: Roman Kagan <rkagan at virtuozzo.com>
>> VZ-Bug: PSBM-79523
>> ---
>>  drivers/virtio/virtio_ring.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> index d77a91f39d55..5903139aede2 100644
>> --- a/drivers/virtio/virtio_ring.c
>> +++ b/drivers/virtio/virtio_ring.c
>> @@ -275,7 +275,6 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>>  	}
>>  #endif
>>  
>> -	BUG_ON(total_sg > vq->vring.num);
>>  	BUG_ON(total_sg == 0);
>>  
>>  	head = vq->free_head;
>> @@ -284,8 +283,10 @@ static inline int virtqueue_add(struct virtqueue *_vq,
>>  	 * buffers, then go indirect. FIXME: tune this threshold */
>>  	if (vq->indirect && total_sg > 1 && vq->vq.num_free)
>>  		desc = alloc_indirect(_vq, total_sg, gfp);
>> -	else
>> +	else {
>>  		desc = NULL;
>> +		WARN_ON_ONCE(total_sg > vq->vring.num && !vq->indirect);
>> +	}
>>  
>>  	if (desc) {
>>  		/* Use a single buffer which doesn't continue */
> this is 120% not enough. vq->indirect is not enabled for
> virtblock/virt-scsi.
> Thus we should turn that on in guest (the feature is advertized) and
> only after that we will see true performance improvement.
>
> Den
pls disregard this. I have misinterpreted mainstream sources.

Den