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SubjectRe: [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@redhat.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@oracle.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@redhat.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

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