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From:   Yongji Xie <xieyongji@bytedance.com>
Date:   Tue, 29 Jun 2021 10:26:40 +0800
Message-ID: <CACycT3u9-id2DxPpuVLtyg4tzrUF9xCAGr7nBm=21HfUJJasaQ@mail.gmail.com>
Subject: Re: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace
To:     Jason Wang <jasowang@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        Stefano Garzarella <sgarzare@redhat.com>,
        Parav Pandit <parav@nvidia.com>,
        Christoph Hellwig <hch@infradead.org>,
        Christian Brauner <christian.brauner@canonical.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Matthew Wilcox <willy@infradead.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Jens Axboe <axboe@kernel.dk>, bcrl@kvack.org,
        Jonathan Corbet <corbet@lwn.net>,
        =?UTF-8?Q?Mika_Penttil=C3=A4?= <mika.penttila@nextfour.com>,
        Dan Carpenter <dan.carpenter@oracle.com>, joro@8bytes.org,
        Greg KH <gregkh@linuxfoundation.org>, songmuchun@bytedance.com,
        virtualization <virtualization@lists.linux-foundation.org>,
        netdev@vger.kernel.org, kvm <kvm@vger.kernel.org>,
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On Mon, Jun 28, 2021 at 12:40 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> =E5=9C=A8 2021/6/25 =E4=B8=8B=E5=8D=8812:19, Yongji Xie =E5=86=99=E9=81=
=93:
> >> 2b) for set_status(): simply relay the message to userspace, reply is =
no
> >> needed. Userspace will use a command to update the status when the
> >> datapath is stop. The the status could be fetched via get_stats().
> >>
> >> 2b looks more spec complaint.
> >>
> > Looks good to me. And I think we can use the reply of the message to
> > update the status instead of introducing a new command.
> >
>
> Just notice this part in virtio_finalize_features():
>
>          virtio_add_status(dev, VIRTIO_CONFIG_S_FEATURES_OK);
>          status =3D dev->config->get_status(dev);
>          if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {
>
> So we no reply doesn't work for FEATURES_OK.
>
> So my understanding is:
>
> 1) We must not use noreply for set_status()
> 2) We can use noreply for get_status(), but it requires a new ioctl to
> update the status.
>
> So it looks to me we need synchronize for both get_status() and
> set_status().
>

We should not send messages to userspace in the FEATURES_OK case. So
the synchronization is not necessary.

Thanks,
Yongji