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

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Message-ID: <20220524075158.2vyuw7ga72xub7pp@sgarzare-redhat>
Date:   Tue, 24 May 2022 09:51:58 +0200
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Eugenio Perez Martin <eperezma@...hat.com>
Cc:     Si-Wei Liu <si-wei.liu@...cle.com>,
        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, 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 Tue, May 24, 2022 at 09:42:06AM +0200, Eugenio Perez Martin wrote:
>On Tue, May 24, 2022 at 9:09 AM Stefano Garzarella <sgarzare@...hat.com> wrote:
>>
>> On Mon, May 23, 2022 at 09:20:14PM +0200, Eugenio Perez Martin wrote:
>> >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.
>>
>> For block devices wait for all in-flight requests could take several
>> time.
>>
>> Could this be a problem if the caller gets stuck on this ioctl?
>>
>> If it could be a problem, maybe we should use an eventfd to signal that
>> the device is successfully stopped.
>>
>
>For that particular problem I'd very much prefer to add directly an
>ioctl to get the inflight descriptors. We know for sure we will need
>them, and it will be cleaner in the long run.

Makes sense!

>
>As I understand the vdpa block simulator, there is no need to return
>the inflight descriptors since all of the requests are processed in a
>synchronous way. So, for this iteration, we could offer the stop
>feature to qemu.

Right, the simulator handles everything synchronously.

>
>Other non-simulated devices would need it. Could it be delayed to
>future development?

Yep, sure, it sounds like you already have a plan, so no problem :-)

Thanks,
Stefano