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

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?

devices sequentially increases migration downtime, so I think the
interface should encourage concurrently stopping multiple devices.

I think you and Cornelia discussed that an interrupt could be added to
solve this problem. That would address my concerns about the STOP bit.

The problems are:

1) if we generate an interrupt after STOP, it breaks the STOP semantic where the device should not generate any interrupt 2) if we generate an interrupt before STOP, we may end up with race conditions



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