Re: [Qemu-devel] [PATCH 09/10] virtio: read avail_idx from VQ only when necessary

[Paolo Bonzini]

On 19/01/2016 17:54, Michael S. Tsirkin wrote:
> On Fri, Jan 15, 2016 at 01:41:57PM +0100, Paolo Bonzini wrote:
>> From: Vincenzo Maffione <address@hidden>
>>
>> The virtqueue_pop() implementation needs to check if the avail ring
>> contains some pending buffers. To perform this check, it is not
>> always necessary to fetch the avail_idx in the VQ memory, which is
>> expensive. This patch introduces a shadow variable tracking avail_idx
>> and modifies virtio_queue_empty() to access avail_idx in physical
>> memory only when necessary.
>>
>> Signed-off-by: Vincenzo Maffione <address@hidden>
>> Message-Id: <address@hidden>
>> Signed-off-by: Paolo Bonzini <address@hidden>
> 
> Is the cost due to the page walk?

Yes, as with all the other patches.  But unlike patches 7 and 10 where
we just reduce the number of walks, for patch 8 and 9 it's difficult to
beat a local cache. :)

>> @@ -1579,6 +1595,7 @@ int virtio_load(VirtIODevice *vdev, QEMUFile *f, int 
>> version_id)
>>                  return -1;
>>              }
>>              vdev->vq[i].used_idx = vring_used_idx(&vdev->vq[i]);
>> +            vdev->vq[i].shadow_avail_idx = vring_avail_idx(&vdev->vq[i]);
>>          }
>>      }
> 
> 
> shadow_avail_idx also should be updated on vhost stop,

That's virtio_queue_set_last_avail_idx, right?

Paolo