Re: [Qemu-devel] ResettRe: [Xen-devel] [v5][PATCH 0/5] xen: add Intel IGD passthrough support

[Michael S. Tsirkin]

On Fri, Jul 04, 2014 at 08:28:25AM +0200, Paolo Bonzini wrote:
> Il 03/07/2014 21:09, Jesse Barnes ha scritto:
> >Practically speaking, we could probably assume specific CPU/PCH combos,
> >as I don't think they're generally mixed across generations, though SNB
> >and IVB did have compatible sockets, so there is the possibility of
> >mixing CPT and PPT PCHs, but those are register identical as far as the
> >graphics driver is concerned, so even that should be safe.
> 
> I guess the driver could do that if it finds an unknown PCH device ID.

I would say if possible, do this unconditionally.
If this logic is strictly required then I would
also check the subsystem vendor id and skip the
PCH tricks if it matches Xen.

> But
> encoding it in the subsystem device ID could also work and it would be easy
> to do in the hypervisor.

Right, but that's custom code in the hypervisor as opposed to the generic one.
If generic one can work, that's much better.

> >Beyond that, the other MCH data we need to look at is mirrored into the
> >GPU's MMIO space on current gens.
> 
> Heh, that's exactly the same as the paravirtualized solution we were
> suggesting. ;)
> 
> Paolo
> 
> >On older gens, we do need to poke at
> >the memory controller a bit to get some info (see
> >intel_setup_mchbar()), but that's not true of newer stuff.  Looks like
> >we only short circuit that on VLV though; we could probably do it on
> >SNB+.