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

[Jason Wang <jasowang@redhat.com>]

å 2021/7/19 äå8:18, Stefan Hajnoczi åé:
> On Fri, Jul 16, 2021 at 09:44:26AM +0800, Jason Wang wrote:
> > å 2021/7/15 äå5:16, Stefan Hajnoczi åé:
> > > On Thu, Jul 15, 2021 at 09:35:13AM +0800, Jason Wang wrote:
> > > > å 2021/7/14 äå11:07, Stefan Hajnoczi åé:
> > > > > On Wed, Jul 14, 2021 at 06:29:28PM +0800, Jason Wang wrote:
> > > > > > And as I've stated several times, virtqueue is the interface or transport
> > > > > > which carries the commands for implementing specific semantics. It doesn't
> > > > > > conflict with what is proposed in this patch.
> > > > > The abstract operations for stopping the device and fetching virtqueue
> > > > > state sound good to me, but I don't think a Device Status field STOP bit
> > > > > should be added. An out-of-band stop operation would support devices
> > > > > that take a long time to stop better.
> > > > So the long time request is not something that is introduced by the STOP
> > > > bit. Spec already use that for reset.
> > > Reset doesn't affect migration downtime. The register polling approach
> > > is problematic during migration downtime because it's difficult to stop
> > > multiple devices and do other downtime cleanup concurrently.
> >
> > This part I don't understand. We don't have an centralized control path that
> > is used for each virtual functions.
> >
> > VMM is free to stop multiple devices and poll for all those device status?
> Yes, it's possible to do that but I think it's harder for VMMs to
> implement and consumes CPU (which competes with software devices that
> are trying to stop).


Possibly, actually, there are two issues:

1) How to send the command to check the succeeding of the command
2) Whether or not we need a notification for the completion of the command

What is being proposed is 1), and 2) could be done via a transport 
specific way. (But requires more thought).

Actually, it's the software that choose the best way for them, it can do 
busy polling, timer etc.

Thanks


> Stefan