netdev - Re: Re: [PATCH v8 09/10] vduse: Introduce VDUSE

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Date:   Tue, 29 Jun 2021 11:56:27 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Parav Pandit <parav@...dia.com>,
        Christoph Hellwig <hch@...radead.org>,
        Christian Brauner <christian.brauner@...onical.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>, bcrl@...ck.org,
        Jonathan Corbet <corbet@....net>,
        Mika Penttilä <mika.penttila@...tfour.com>,
        Dan Carpenter <dan.carpenter@...cle.com>, joro@...tes.org,
        Greg KH <gregkh@...uxfoundation.org>, songmuchun@...edance.com,
        virtualization <virtualization@...ts.linux-foundation.org>,
        netdev@...r.kernel.org, kvm <kvm@...r.kernel.org>,
        linux-fsdevel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Re: [PATCH v8 09/10] vduse: Introduce VDUSE - vDPA Device in Userspace

On Tue, Jun 29, 2021 at 11:29 AM Jason Wang <jasowang@...hat.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@...hat.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