Re: [Qemu-devel] [PATCH] e1000/rtl8139: forbid dealing with packets when VM is paused

[Stefan Hajnoczi]

On Mon, May 26, 2014 at 02:51:48PM +0300, Michael S. Tsirkin wrote:
> On Mon, May 26, 2014 at 01:48:13PM +0200, Stefan Hajnoczi wrote:
> > On Wed, May 14, 2014 at 03:46:48PM +0300, Michael S. Tsirkin wrote:
> > > On Wed, May 14, 2014 at 02:30:26PM +0200, Stefan Hajnoczi wrote:
> > > > On Thu, May 08, 2014 at 12:51:05PM +0000, Zhanghailiang wrote:
> > > > > > If you implement this in the net layer then that problem is easy to 
> > > > > > resolve since
> > > > > > we can flush all queues when the guest resumes to get packets 
> > > > > > flowing again.
> > > > > > 
> > > > > Do you mean we should also listen for VM runstate changes in net 
> > > > > layer, and when detect runstate changes back to running , we flush 
> > > > > all queues actively? Am I misunderstanding? 
> > > > > Or we can do it *before* qemu (exactly when it check if it can send 
> > > > > packets) send packets to guest again, this way will be simple, but it 
> > > > > also need know the change of runstate. Any idea? 
> > > > 
> > > > When the runstate changes back to running, we definitely need to flush
> > > > queues to get packets flowing again.  I think the simplest way of doing
> > > > that is in the net layer so individual NICs and netdevs don't have to
> > > > duplicate this code.
> > > > 
> > > > Stefan
> > > 
> > > That will help with networking but not other devices.
> > > The issue isn't limited to networking at all.
> > > How about we stop all io threads with the vm?
> > > 
> > > That will address the issue in a generic way.
> > 
> > I'm not sure if it works in all cases, for example iSCSI where we send
> > nop keepalives.
> > 
> > Stefan
> 
> I am guessing that runs from the realtime clock?
> We definitely want to keep realtime clock going when VM
> is stopped, that's the definition.

Yes.  And this is a random example I picked but there are probably more
cases.

I think stopping event loops could be a good idea, but we need to audit
the code carefully and probably introduce callbacks to pause things
cleanly instead of just suspending threads.

Stefan