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 2021/7/13 äå4:19, Cornelia Huck åé:
>> 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.)
>
>
> Yes, that's my understanding.
>
>
>>
>> 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.
>
>
> A question, is interrupt used for such notification, or the VMM can 
> choose to poll for the completion?

You can poll for the subchannel to become status pending.

>
>
>> Not sure how
>> well that translates to other transports.
>
>
> Actually, it's not necessarily a busy polling. VMM can schedule other 
> process in and recheck the bit periodically.
>
> Or as you mentioned before, we can use some kind of interrupt but it 
> would be more complicated than the simple status bit. It's better to 
> introduce the interrupt only if the status bit doesn't fit.

At least for ccw, waiting for the status bit to be set also involves an
interrupt or polling (we use another channel program to retrieve the
status.) A dedicated channel command would actually be better, as the
interrupt/status pending would already inform us of success.