Re: [Qemu-devel] [PATCH 05/11] qemu: MSI-X support functions

[Michael S. Tsirkin]

On Wed, Jun 10, 2009 at 06:30:16PM +0100, Paul Brook wrote:
> On Wednesday 10 June 2009, Michael S. Tsirkin wrote:
> > On Wed, Jun 10, 2009 at 05:46:03PM +0100, Paul Brook wrote:
> > > > > If you can't create an identical machine from scratch then I don't
> > > > > consider snapshot/migration to be a useful feature. i.e. as soon as
> > > > > you shutdown and restart the guest it is liable to break anyway.
> > > >
> > > > Why is liable to break?
> > >
> > > A VM booted on an old version of qemu and migrated to a new version will
> > > behave differently to a the same VM booted on a new version of qemu.
> >
> > It will behave identically. That's what the patch does: discover
> > how did the device behave on old qemu, and make it behave the same way
> > on new qemu.
> 
> You're missing the point.  After doing a live migration from old-qemu to new-
> qemu, there is no snapshot to load.  We need to be able to shutdown the 
> guest, 
> kill qemu (without saving a snapshot), then start qemu with the exact same 
> hardware.

Yes, I see how this would sometimes be useful. So this feature request
would mean that we need a flag to disable msix in virtio net.

I think we can agree that hardware can change across reboots. It can
with real hardware and it happens without guest doing anything.

> If we can't start a new qemu with the same hardware configuration then we 
> should not be allowing migration or loading of snapshots.
> 
> Paul

OK, so I'll add an option in virtio-net to disable msi-x, and such
an option will be added in any device with msi-x support.
Will that address your concern?

-- 
MST