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

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Message-ID: <CACycT3vpMFbc9Fzuo9oksMaA-pVb1dEVTEgjNoft16voryPSWQ@mail.gmail.com>
Date:   Fri, 25 Jun 2021 12:19:15 +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 Fri, Jun 25, 2021 at 11:09 AM Jason Wang <jasowang@...hat.com> wrote:
>
>
> 在 2021/6/24 下午5:16, Yongji Xie 写道:
> > On Thu, Jun 24, 2021 at 4:14 PM Jason Wang <jasowang@...hat.com> wrote:
> >>
> >> 在 2021/6/24 下午12:46, Yongji Xie 写道:
> >>>> So we need to deal with both FEATURES_OK and reset, but probably not
> >>>> DRIVER_OK.
> >>>>
> >>> OK, I see. Thanks for the explanation. One more question is how about
> >>> clearing the corresponding status bit in get_status() rather than
> >>> making set_status() fail. Since the spec recommends this way for
> >>> validation which is done in virtio_dev_remove() and
> >>> virtio_finalize_features().
> >>>
> >>> Thanks,
> >>> Yongji
> >>>
> >> I think you can. Or it would be even better that we just don't set the
> >> bit during set_status().
> >>
> > Yes, that's what I mean.
> >
> >> I just realize that in vdpa_reset() we had:
> >>
> >> static inline void vdpa_reset(struct vdpa_device *vdev)
> >> {
> >>           const struct vdpa_config_ops *ops = vdev->config;
> >>
> >>           vdev->features_valid = false;
> >>           ops->set_status(vdev, 0);
> >> }
> >>
> >> We probably need to add the synchronization here. E.g re-read with a
> >> timeout.
> >>
> > Looks like the timeout is already in set_status().
>
>
> Do you mean the VDUSE's implementation?
>

Yes.

>
> >   Do we really need a
> > duplicated one here?
>
>
> 1) this is the timeout at the vDPA layer instead of the VDUSE layer.

OK, I get it.

> 2) it really depends on what's the meaning of the timeout for set_status
> of VDUSE.
>
> Do we want:
>
> 2a) for set_status(): relay the message to userspace and wait for the
> userspace to quiescence the datapath
>
> or
>
> 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.

> > And how to handle failure? Adding a return value
> > to virtio_config_ops->reset() and passing the error to the upper
> > layer?
>
>
> Something like this.
>

OK.

Thanks,
Yongji