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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

-- 
MST