Re: [virtio-comment] [PATCH V2 2/2] virtio: introduce STOP status bit

[Cornelia Huck <cohuck@redhat.com>]

On Tue, Jul 13 2021, Jason Wang <jasowang@redhat.com> wrote:

> å 2021/7/12 äå5:57, Stefan Hajnoczi åé:
>> When migrating a guest with many VIRTIO devices a busy waiting approach
>> extends downtime if implemented sequentially (stopping one device at a
>> time).
>
>
> Well. You need some kinds of waiting for sure, the device/DMA needs 
> sometime to be stopped. The downtime is determined by a specific virtio 
> implementation which is hard to be restricted at the spec level. We can 
> clarify that the device must set the STOP bit in e.g 100ms.

I don't think we can introduce arbitrary upper bounds here. At most, we
can say that the device SHOULD try to set the STOP bit as early as
possible (and make use of the mechanism to expose in-flight buffers.)

If we want to avoid polling for the STOP bit, we need some kind of
notification mechanism, I guess. For ccw, I'd just use a channel
command to stop the device; completion of that channel program would
indicate that the device is done with the stop procedure. Not sure how
well that translates to other transports.