Re: [Qemu-devel] [PATCH] hw/virtio/virtio.c: fix the vring_avail_event error

[Bin Wu <wu.wubin@huawei.com>]

On 2014/10/28 13:32, Michael S. Tsirkin wrote:
> On Tue, Oct 28, 2014 at 02:13:02AM +0000, Bin Wu wrote:
>> The event idx in virtio is an effective way to reduce the number of
>> interrupts and exits of the guest. When the guest puts an request
>> into the virtio ring, it doesn't exit immediately to inform the
>> backend. Instead, the guest checks the "avail" event idx to determine
>> the notification.
>>
>> In virtqueue_pop, when a request is poped, the current avail event
>> idx should be set to the number of vq->last_avail_idx.
>>
>> Signed-off-by: Bin Wu <wu.wubin@huawei.com>
> 
> Does this fix some observable bug? Improve efficiency for some workload?
>
Improve efficiency, I test as follows:
(1)create a suse11sp3 VM with a virtio-blk test disk;
(2)use libaio library to submit 32 IOs concurrently (invoke io_sumbmit 32 times)
in the VM;
(3)modify the virtio-blk driver in the guest to record the host notification times;

without this patch, virtio-blk may notify the host between 10 and 30 times;
with this patch, virtio-blk just notify the host once during every test:)

>> ---
>>  hw/virtio/virtio.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
>> index 2c236bf..013979a 100644
>> --- a/hw/virtio/virtio.c
>> +++ b/hw/virtio/virtio.c
>> @@ -469,7 +469,7 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
>>  
>>      i = head = virtqueue_get_head(vq, vq->last_avail_idx++);
>>      if (vdev->guest_features & (1 << VIRTIO_RING_F_EVENT_IDX)) {
>> -        vring_avail_event(vq, vring_avail_idx(vq));
>> +        vring_avail_event(vq, vq->last_avail_idx);
>>      }
>>  
>>      if (vring_desc_flags(vdev, desc_pa, i) & VRING_DESC_F_INDIRECT) {
>> -- 
>> 1.7.12.4
> 
> .
> 

-- 
Bin Wu