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

[Stefan Hajnoczi <stefanha@redhat.com>]

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).

Stefan

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