Re: [PATCH] cast used->idx to volatile

[Linhaifeng <haifeng.lin-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>]

On 2015/3/23 20:54, Xie, Huawei wrote:
> 
> 
>> -----Original Message-----
>> From: Linhaifeng [mailto:haifeng.lin-hv44wF8Li93QT0dZR+AlfA@public.gmane.org]
>> Sent: Monday, March 23, 2015 8:24 PM
>> To: dev-VfR2kkLFssw@public.gmane.org
>> Cc: Ouyang, Changchun; Xie, Huawei
>> Subject: Re: [dpdk-dev] [PATCH] cast used->idx to volatile
>>
>>
>>
>> On 2015/3/21 16:07, linhaifeng wrote:
>>> From: Linhaifeng <haifeng.lin-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
>>>
>>> Same as rte_vhost_enqueue_burst we should cast used->idx
>>> to volatile before notify guest.
>>>
>>> Signed-off-by: Linhaifeng <haifeng.lin-hv44wF8Li93QT0dZR+AlfA@public.gmane.org>
>>> ---
>>>  lib/librte_vhost/vhost_rxtx.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/lib/librte_vhost/vhost_rxtx.c b/lib/librte_vhost/vhost_rxtx.c
>>> index 535c7a1..8d674d1 100644
>>> --- a/lib/librte_vhost/vhost_rxtx.c
>>> +++ b/lib/librte_vhost/vhost_rxtx.c
>>> @@ -722,7 +722,7 @@ rte_vhost_dequeue_burst(struct virtio_net *dev,
>> uint16_t queue_id,
>>>  	}
>>>
>>>  	rte_compiler_barrier();
>>> -	vq->used->idx += entry_success;
>>> +	*(volatile uint16_t *)&vq->used->idx += entry_success;
> 
> 
> Haifeng:
> We have compiler barrier before and an external function call behind, so we don't need volatile  here.
> Do you meet issue?
> 

Tx_q is sometimes stopped when we use virtio_net. Because vhost thought there are no buffers in tx_q and virtio_net
though vhost haven't handle all packets so we have to restart VM to restore work.

The status in VM is:
Mar 18 17:11:10 linux-b2ij kernel: [46337.246687] net eth7: virtnet_poll
Mar 18 17:11:10 linux-b2ij kernel: [46337.246690] net eth7: receive_buf
Mar 18 17:11:10 linux-b2ij kernel: [46337.246693] net eth7: vi->num=239
Mar 18 17:11:10 linux-b2ij kernel: [46337.246695] net eth7: svq:avail->idx=52939 used->idx=52939 num_free=18 num_added=0 svq->last_used_idx=52820
Mar 18 17:11:10 linux-b2ij kernel: [46337.246699] net eth7: rvq:avail->idx=36215 used->idx=35977 num_free=18 num_added=0 rvq->last_used_idx=35977
Mar 18 17:11:11 linux-b2ij kernel: [46337.901038] net eth7: dev_queue_xmit, qdisc->flags=4, qdisc->state deactiveed=0
Mar 18 17:11:12 linux-b2ij kernel: [46337.901042] net eth7: dev_queue_xmit, txq->state=1, stopped=1

Why compiler barrier not take effect in our case? Is compiler barrier depended on -O3 option? We use -O2 option.

>>>  	/* Kick guest if required. */
>>>  	if (!(vq->avail->flags & VRING_AVAIL_F_NO_INTERRUPT))
>>>  		eventfd_write((int)vq->callfd, 1);
>>>
>>
>> --
>> Regards,
>> Haifeng
> 
> 
> 

-- 
Regards,
Haifeng