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 下午1:48, Yongji Xie wrote:
> On Wed, 30 Jan 2019 at 10:32, Jason Wang <jasowang@redhat.com> 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.
>>
> The cache line should have been flushed during crash. So we can see
> the correct value when backend reconnecting. If so, compile barrier
> should be enough here, right?


Maybe I worry too much but it's not about flushing cache, but about 
whether or not compiler can generate mov to memory instead of mov to 
registers.

Thanks


>
> Thanks,
> Yongji