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From: "Xie, Huawei" <huawei.xie@intel.com>
To: Yuanhan Liu <yuanhan.liu@linux.intel.com>
CC: "dev@dpdk.org" <dev@dpdk.org>, "stephen@networkplumber.org"
<stephen@networkplumber.org>, "Ananyev, Konstantin"
<konstantin.ananyev@intel.com>, "thomas.monjalon@6wind.com"
<thomas.monjalon@6wind.com>
Thread-Topic: [dpdk-dev] [PATCH] virtio: use volatile to get used->idx in
the loop
Thread-Index: AdG7yA4yoCOUROu/Rf+v+VEjIsQC1w==
Date: Thu, 2 Jun 2016 08:39:36 +0000
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Subject: Re: [dpdk-dev] [PATCH] virtio: use volatile to get used->idx in the
loop
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On 6/1/2016 2:03 PM, Yuanhan Liu wrote:=0A=
> On Wed, Jun 01, 2016 at 05:40:08AM +0000, Xie, Huawei wrote:=0A=
>> On 5/30/2016 4:20 PM, Yuanhan Liu wrote:=0A=
>>> On Wed, May 25, 2016 at 12:16:41AM +0800, Huawei Xie wrote:=0A=
>>>> There is no external function call or any barrier in the loop,=0A=
>>>> the used->idx would only be retrieved once.=0A=
>>>>=0A=
>>>> Signed-off-by: Huawei Xie <huawei.xie@intel.com>=0A=
>>>> ---=0A=
>>>> drivers/net/virtio/virtio_ethdev.c | 3 ++-=0A=
>>>> 1 file changed, 2 insertions(+), 1 deletion(-)=0A=
>>>>=0A=
>>>> diff --git a/drivers/net/virtio/virtio_ethdev.c b/drivers/net/virtio/v=
irtio_ethdev.c=0A=
>>>> index c3fb628..f6d6305 100644=0A=
>>>> --- a/drivers/net/virtio/virtio_ethdev.c=0A=
>>>> +++ b/drivers/net/virtio/virtio_ethdev.c=0A=
>>>> @@ -204,7 +204,8 @@ virtio_send_command(struct virtqueue *vq, struct v=
irtio_pmd_ctrl *ctrl,=0A=
>>>> usleep(100);=0A=
>>>> }=0A=
>>>> =0A=
>>>> - while (vq->vq_used_cons_idx !=3D vq->vq_ring.used->idx) {=0A=
>>>> + while (vq->vq_used_cons_idx !=3D=0A=
>>>> + *((volatile uint16_t *)(&vq->vq_ring.used->idx))) {=0A=
>>> I'm wondering maybe we could fix VIRTQUEUE_NUSED (which has no such=0A=
>>> qualifier) and use this macro here?=0A=
>>>=0A=
>>> If you check the reference of that macro, you might find similar=0A=
>>> issues, say, it is also used inside the while-loop of=0A=
>>> virtio_recv_mergeable_pkts().=0A=
>>>=0A=
>>> --yliu=0A=
>>> =0A=
>>>=0A=
>> Yes, seems it has same issue though haven't confirmed with asm code.=0A=
> So, move the "volatile" qualifier to VIRTQUEUE_NUSED?=0A=
>=0A=
> --yliu=0A=
>=0A=
=0A=
Yes, anyway this is just intermediate fix. In next patch, will declare=0A=
the idx as volatile, and remove the qualifier in the macro.=0A=