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

[Yongji Xie]

On Fri, 1 Feb 2019 at 10:28, Jason Wang <address@hidden> wrote:
>
>
> On 2019/1/30 下午1:48, Yongji Xie wrote:
> > On Wed, 30 Jan 2019 at 10:32, Jason Wang <address@hidden> wrote:
> >>
> >> On 2019/1/22 下午4:31, address@hidden 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.
>

OK, I see. I will declare those variables as volatile in v6. Thank you.

Thanks,
Yongji