Re: [Qemu-devel] [PATCH v3 11/11] igd: move igd-passthrough-isa-bridge creation to machine init

[Stefano Stabellini]

On Fri, 8 Jan 2016, Stefano Stabellini wrote:
> > > > xen_pt_initfn checks that igd-passthru=on is set in case it finds a igd
> > > > device is assigned, that will make sure the igd-isa-bridge is present.
> > > > 
> > > > But, yes, you can create a igd-isa-bridge now even when not assigning a
> > > > igd device, either by specifying igd-passthru=on or using -device.  I
> > > > fail to see why this is a problem though, care to explain?
> > > 
> > > It is going to change the PCI layout of any virtual machines with a
> > > config file containing
> > > 
> > > gfx_passthru="igd"
> > > 
> > > and no pci config line. A Xen 4.7 user could add gfx_passthru="igd" to
> > > all her VM config files, because actually it does nothing unless an
> > > Intel graphic card is assigned to the VM.
> > 
> > No.  It changes the host bridge even when not passing through a igd,
> > because that is linked to igd-passthru=on only.
> >
> > So making both host bridge tweak and isa bridge tweak triggered by
> > igd-passthru=on brings more consistency to the whole thing.
> 
> That is true. Given that the only qemu-xen codebase with igd support is
> 4.7 and 4.7 hasn't been released yet, I am OK with changing the guest
> visible PCI layout. I might ask for your help in backporting the patches
> ;-)

One thing that I forgot to consider is that QEMU 2.5 has been released
with igd passthrough too and Xen 4.6 + QEMU 2.5 is a combination we
should support.

However QEMU 2.5 has a serious bug
(http://marc.info/?l=qemu-devel&m=145172165010604) which probably
prevents igd passthrough from working at all. I asked Xudong to
investigate. I am thinking that if the feature works in 2.5, we need to
support it, therefore we cannot break migration by changing the PCI
layout.  Otherwise if the feature doesn't work, we could take the
liberty to make the change.  Do you agree?