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Date: Tue, 29 Jan 2019 22:14:56 -0500
From: "Michael S. Tsirkin" <mst@redhat.com>
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References: <20190122083152.10705-1-xieyongji@baidu.com>
	<20190122083152.10705-4-xieyongji@baidu.com>
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Subject: Re: [Qemu-devel] [PATCH v5 3/6] libvhost-user: Support tracking
 inflight I/O in shared memory
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To: Jason Wang <jasowang@redhat.com>
Cc: elohimes@gmail.com, stefanha@gmail.com, marcandre.lureau@redhat.com, berrange@redhat.com, maxime.coquelin@redhat.com, yury-kotov@yandex-team.ru, wrfsh@yandex-team.ru, nixun@baidu.com, qemu-devel@nongnu.org, lilin24@baidu.com, zhangyu31@baidu.com, chaiwen@baidu.com, Xie Yongji <xieyongji@baidu.com>

On Wed, Jan 30, 2019 at 10:31:49AM +0800, Jason Wang wrote:
>=20
> On 2019/1/22 =E4=B8=8B=E5=8D=884:31, 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 =3D 1;
> > +
> > +    vq->inflight->desc[desc_idx].avail_idx =3D 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 =3D vq->used_idx;
> > +
> > +    barrier();
> > +
> > +    vq->inflight->desc[desc_idx].version++;
> > +
> > +    return 0;
> > +}
>=20
>=20
> You probably need WRITE_ONCE() semantic (e.g volatile) to make sure the
> value reach memory.
>=20
> Thanks
>=20

WRITE_ONCE is literally volatile + dependency memory barrier.
So unless compiler is a very agressive one, it does not
buy you much.

--=20
MST