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In-Reply-To: <e82766ff-dc6b-2cbb-3504-0ef618d538e2@redhat.com>
From:   Yongji Xie <xieyongji@bytedance.com>
Date:   Tue, 29 Jun 2021 11:56:27 +0800
Message-ID: <CACycT3ucVz3D4Tcr1C6uzWyApZy7Xk4o17VH2gvLO3w1Ra+skg@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>,
        linux-fsdevel@vger.kernel.org, iommu@lists.linux-foundation.org,
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On Tue, Jun 29, 2021 at 11:29 AM Jason Wang <jasowang@redhat.com> wrote:
>
>
> =E5=9C=A8 2021/6/29 =E4=B8=8A=E5=8D=8810:26, Yongji Xie =E5=86=99=E9=81=
=93:
> > 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 i=
s 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.
>
>
> As discussed previously, there could be a device that mandates some
> features (VIRTIO_F_RING_PACKED). So it can choose to not accept
> FEATURES_OK is packed virtqueue is not negotiated.
>
> In this case we need to relay the message to userspace.
>

OK, I see. If so, I prefer to only use noreply for set_status(). We do
not set the status bit if the message is failed. In this way, we don't
need to change lots of virtio core codes to handle the failure of
set_status()/get_status().

Thanks,
Yongji