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    SubjectRe: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace
    On Tue, Jun 29, 2021 at 11:29 AM Jason Wang <jasowang@redhat.com> wrote:
    >
    >
    > 在 2021/6/29 上午10:26, Yongji Xie 写道:
    > > On Mon, Jun 28, 2021 at 12:40 PM Jason Wang <jasowang@redhat.com> wrote:
    > >>
    > >> 在 2021/6/25 下午12:19, Yongji Xie 写道:
    > >>>> 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 = 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

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