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 [off-list ref] wrote:

quoted

On Mon, May 23, 2022 at 09:20:14PM +0200, Eugenio Perez Martin wrote:

quoted

On Sat, May 21, 2022 at 12:13 PM Si-Wei Liu [off-list ref] wrote:

quoted

On 5/20/2022 10:23 AM, Eugenio Pérez wrote:

quoted

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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