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

[Paolo Bonzini]

Il 25/06/2014 11:50, Chen, Tiejun ha scritto:
> > For past devices, we know which BARs they use.  For future devices, it
> > would be nice if the PCH/MCH backdoor was specified so that we know they
> > will leave a free BAR for virtualization.
>
> Now I'm a bit confused about BAR here.
>
> You're saying we will reserve a free BAR to address those information to
> expose to guest, but which device does this free BAR belong to? The
> video device? Or PCH/MCH?

The video device.  If the host device does not have the BAR (which will 
be the common case), QEMU can emulate it like this:

- offsets 0x0000..0x0fff map to configuration space of the host MCH

- offsets 0x1000..0x1fff map to configuration space of the host PCH

Of course this is only limited to offsets that are needed by the driver.

Paolo