Re: [Qemu-devel] [PATCH rebased for-1.8] i386: pc: align gpa<->hpa on 1GB boundary (v6)

[Gerd Hoffmann]

On Di, 2013-12-10 at 15:58 +0100, Paolo Bonzini wrote:
> Il 10/12/2013 15:53, Gerd Hoffmann ha scritto:
> >   Hi,
> > 
> >> If we could make a small guset visible change, it would be simpler to
> >> always make the PCI hole 1GB in size; it is currently 256MB for i440FX
> >> and 1.25GB for q35.
> > 
> > Easy for i440fx.
> > 
> > Tricky for q35 as the firmware knows qemu will not map ram above
> > 0xb000000 and places the mmconfig bar @ 0xb0000000.  Making the window
> > smaller (1.25GB -> 1GB) will create a conflict there.  Making it larger
> > (2G) will work.  It's done this way to keep 0xc0000000+ free for pci
> > bars, and we can map up to 512MB-sized bars there.
> 
> 0xc0000000-0xfebfffff is almost 1GB, so there is room for 1 512MB-size
> BAR.  mmconfig could move to 0xf0000000-0xf7ffffff;

Reduces number of pci busses from 256 to 128.  Not that this is a
problem now, but something to consider to make sure things are
future-proof.

> firmware is not a
> problem because this would be only for new machine types.

Even on qemu 2.0 it is the firmware which decides where to place the
mmconfig bar.  qemu will look at northbridge xbar register to generate a
matching mcfg acpi table.

cheers,
  Gerd