Re: [Qemu-devel] [v5][PATCH 4/5] xen, gfx passthrough: create host bridge to passthrough

[Michael S. Tsirkin]

On Sun, Jun 29, 2014 at 03:56:10PM +0800, Chen, Tiejun wrote:
> On 2014/6/27 19:26, Paolo Bonzini wrote:
> >Il 27/06/2014 10:34, Chen, Tiejun ha scritto:
> >>
> >>
> >>So how to separate this to specific to xen? Or you mean we need to
> >>create an new machine to address this scenario? But actually this is
> >>same as xenfv_machine except for these little codes.
> >
> >Yes, please create a new machine so that "-M pc" doesn't have any of
> >these hacks.
> 
> But regardless of the machine is 'xenfv' or 'pc', we always call
> pc_init_pci(), then inside, i440fx_init() is always performed. So I think
> even we create a new machine, shouldn't we still call pc_init_pci()?
> 
> >
> >Note that "-M xenfv" is obsolete, Xen can now use "-M pc" (i.e. the
> >default).
> >
> 
> Yes, Xen can use 'pc'.
> 
> Thanks
> Tiejun

You are creating a new machine type where the pci host
looks like MCH but a bunch of other things are from i440fx.

I have some doubts about this combination being worth supporting - it
does not seem useful for anything except xen from the code you posted,
but maybe you can whittle down the number of places where you poke at
the host to make it reasonable: I can imagine that, if you are lucky and
the registers that i915 wants to poke to make it work on real hardware
happen to fall on top of reserved registers in the i440FX/PIIX3 pci
bridge.  OTOH it would be much more likely if you just start with
something that does have MCH, like Q35, or emulate a newer
machine type.  This is the path that people who wanted
to boot iOS on QEMU took, and the result is pretty good.

But regardless, this is clearly not a i440fx nor a q35 pc
so it needs a separate name.

-- 
MST