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

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Date:   Tue, 29 Jun 2021 10:26:40 +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 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.

Thanks,
Yongji