Re: [Qemu-devel] [PATCH v5 3/6] libvhost-user: Support tracking inflight I/O in shared memory

[Jason Wang <jasowang@redhat.com>]

On 2019/1/30 上午11:14, Michael S. Tsirkin wrote:
> On Wed, Jan 30, 2019 at 10:31:49AM +0800, Jason Wang wrote:
>> On 2019/1/22 下午4:31, elohimes@gmail.com wrote:
>>> +static int
>>> +vu_queue_inflight_get(VuDev *dev, VuVirtq *vq, int desc_idx)
>>> +{
>>> +    if (!has_feature(dev->protocol_features,
>>> +        VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD)) {
>>> +        return 0;
>>> +    }
>>> +
>>> +    if (unlikely(!vq->inflight)) {
>>> +        return -1;
>>> +    }
>>> +
>>> +    vq->inflight->desc[desc_idx].inuse = 1;
>>> +
>>> +    vq->inflight->desc[desc_idx].avail_idx = vq->last_avail_idx;
>>> +
>>> +    return 0;
>>> +}
>>> +
>>> +static int
>>> +vu_queue_inflight_pre_put(VuDev *dev, VuVirtq *vq, int desc_idx)
>>> +{
>>> +    if (!has_feature(dev->protocol_features,
>>> +        VHOST_USER_PROTOCOL_F_INFLIGHT_SHMFD)) {
>>> +        return 0;
>>> +    }
>>> +
>>> +    if (unlikely(!vq->inflight)) {
>>> +        return -1;
>>> +    }
>>> +
>>> +    vq->inflight->desc[desc_idx].used_idx = vq->used_idx;
>>> +
>>> +    barrier();
>>> +
>>> +    vq->inflight->desc[desc_idx].version++;
>>> +
>>> +    return 0;
>>> +}
>>
>> You probably need WRITE_ONCE() semantic (e.g volatile) to make sure the
>> value reach memory.
>>
>> Thanks
>>
> WRITE_ONCE is literally volatile + dependency memory barrier.
> So unless compiler is a very agressive one, it does not
> buy you much.


Well, since version is increased twice, if compiler decide the inline 
both vu_queue_inflight_pre_put() and vu_queue_inflight_post_put(), can 
we make sure it always generate instructions that write to memory 
instead of registers?

Thanks