linux-kernel - Re: [PATCH 1/4] vdpa: Add stop operation

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Message-ID: <CAJaqyWd3BqZfmJv+eBYOGRwNz3OhNKjvHPiFOafSjzAnRMA_tQ@mail.gmail.com>
Date:   Mon, 23 May 2022 21:20:14 +0200
From:   Eugenio Perez Martin <eperezma@...hat.com>
To:     Si-Wei Liu <si-wei.liu@...cle.com>
Cc:     virtualization <virtualization@...ts.linux-foundation.org>,
        Jason Wang <jasowang@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Stefano Garzarella <sgarzare@...hat.com>,
        Longpeng <longpeng2@...wei.com>,
        Zhu Lingshan <lingshan.zhu@...el.com>,
        Martin Petrus Hubertus Habets <martinh@...inx.com>,
        Harpreet Singh Anand <hanand@...inx.com>, dinang@...inx.com,
        Eli Cohen <elic@...dia.com>,
        Laurent Vivier <lvivier@...hat.com>, pabloc@...inx.com,
        "Dawar, Gautam" <gautam.dawar@....com>,
        Xie Yongji <xieyongji@...edance.com>, habetsm.xilinx@...il.com,
        Christophe JAILLET <christophe.jaillet@...adoo.fr>,
        tanuj.kamde@....com, Wu Zongyong <wuzongyong@...ux.alibaba.com>,
        martinpo@...inx.com, Cindy Lu <lulu@...hat.com>,
        ecree.xilinx@...il.com, Parav Pandit <parav@...dia.com>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Zhang Min <zhang.min9@....com.cn>
Subject: Re: [PATCH 1/4] vdpa: Add stop operation

On Sat, May 21, 2022 at 12:13 PM Si-Wei Liu <si-wei.liu@...cle.com> wrote:
>
>
>
> On 5/20/2022 10:23 AM, Eugenio Pérez wrote:
> > This operation is optional: It it's not implemented, backend feature bit
> > will not be exposed.
> >
> > Signed-off-by: Eugenio Pérez <eperezma@...hat.com>
> > ---
> >   include/linux/vdpa.h | 6 ++++++
> >   1 file changed, 6 insertions(+)
> >
> > diff --git a/include/linux/vdpa.h b/include/linux/vdpa.h
> > index 15af802d41c4..ddfebc4e1e01 100644
> > --- a/include/linux/vdpa.h
> > +++ b/include/linux/vdpa.h
> > @@ -215,6 +215,11 @@ struct vdpa_map_file {
> >    * @reset:                  Reset device
> >    *                          @vdev: vdpa device
> >    *                          Returns integer: success (0) or error (< 0)
> > + * @stop:                    Stop or resume the device (optional, but it must
> > + *                           be implemented if require device stop)
> > + *                           @vdev: vdpa device
> > + *                           @stop: stop (true), not stop (false)
> > + *                           Returns integer: success (0) or error (< 0)
> Is this uAPI meant to address all use cases described in the full blown
> _F_STOP virtio spec proposal, such as:
>
> --------------%<--------------
>
> ...... the device MUST finish any in flight
> operations after the driver writes STOP.  Depending on the device, it
> can do it
> in many ways as long as the driver can recover its normal operation if it
> resumes the device without the need of resetting it:
>
> - Drain and wait for the completion of all pending requests until a
>    convenient avail descriptor. Ignore any other posterior descriptor.
> - Return a device-specific failure for these descriptors, so the driver
>    can choose to retry or to cancel them.
> - Mark them as done even if they are not, if the kind of device can
>    assume to lose them.
> --------------%<--------------
>

Right, this is totally underspecified in this series.

I'll expand on it in the next version, but that text proposed to
virtio-comment was complicated and misleading. I find better to get
the previous version description. Would the next description work?

```
After the return of ioctl, the device MUST finish any pending operations like
in flight requests. It must also preserve all the necessary state (the
virtqueue vring base plus the possible device specific states) that is required
for restoring in the future.

In the future, we will provide features similar to VHOST_USER_GET_INFLIGHT_FD
so the device can save pending operations.
```

Thanks for pointing it out!





> E.g. do I assume correctly all in flight requests are flushed after
> return from this uAPI call? Or some of pending requests may be subject
> to loss or failure? How does the caller/user specify these various
> options (if there are) for device stop?
>
> BTW, it would be nice to add the corresponding support to vdpa_sim_blk
> as well to demo the stop handling. To just show it on vdpa-sim-net IMHO
> is perhaps not so convincing.
>
> -Siwei
>
> >    * @get_config_size:                Get the size of the configuration space includes
> >    *                          fields that are conditional on feature bits.
> >    *                          @vdev: vdpa device
> > @@ -316,6 +321,7 @@ struct vdpa_config_ops {
> >       u8 (*get_status)(struct vdpa_device *vdev);
> >       void (*set_status)(struct vdpa_device *vdev, u8 status);
> >       int (*reset)(struct vdpa_device *vdev);
> > +     int (*stop)(struct vdpa_device *vdev, bool stop);
> >       size_t (*get_config_size)(struct vdpa_device *vdev);
> >       void (*get_config)(struct vdpa_device *vdev, unsigned int offset,
> >                          void *buf, unsigned int len);
>