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

["Xie, Huawei" <huawei.xie@intel.com>]

On 3/24/2015 3:44 PM, Linhaifeng wrote:
>
> On 2015/3/24 9:53, Xie, Huawei wrote:
>> On 3/24/2015 9:00 AM, Linhaifeng wrote:
>>> On 2015/3/23 20:54, Xie, Huawei wrote:
>>>>> -----Original Message-----
>>>>> From: Linhaifeng [mailto:haifeng.lin@huawei.com]
>>>>> Sent: Monday, March 23, 2015 8:24 PM
>>>>> To: dev@dpdk.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@huawei.com>
>>>>>>
>>>>>> Same as rte_vhost_enqueue_burst we should cast used->idx
>>>>>> to volatile before notify guest.
>>>>>>
>>>>>> Signed-off-by: Linhaifeng <haifeng.lin@huawei.com>
>>>>>> ---
>>>>>>  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.
>> compiler barrier always works regardless of the optimization option.
>> I don't get your story, but the key thing is, do you check the asm code?
>> If called from outside as an API, how is it possible it is optimized?
>> there is only one update to used->idx in that function.
>
> Do you mean rte_vhost_enqueue_burst also not need cast used->idx to volatile ? Why not remove it?
I checked the code. Seems we can remove. That is another issue.
For your issue, you meet problem, and submit this this patch, but i am a
bit confused it is the root cause. Do you check the asm code that
volatile is optimized?

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