Re: [Qemu-devel] [PATCH v2] x86: gigabyte alignment for ram

[Gerd Hoffmann]

On Mo, 2014-01-20 at 15:59 +0200, Michael S. Tsirkin wrote:
> On Mon, Jan 20, 2014 at 01:58:45PM +0100, Gerd Hoffmann wrote:
> > On Mo, 2014-01-20 at 13:23 +0200, Michael S. Tsirkin wrote:
> > > On Tue, Dec 17, 2013 at 06:56:06PM +0100, Gerd Hoffmann wrote:
> > > > > I merged your patch but split it: q35 is separate and piix
> > > > > is separate. Would you like me to drop the q35 part then?
> > > > 
> > > > If you are fine with q35 having only 2G lowmem keep it.  It's safe.
> > > > 
> > > > We can sort the mmconfig setup afterwards, then check if (and how) we'll
> > > > transition to 3G lowmem.  Maybe we simply don't after all, with the
> > > > world moving to 64bit it doesn't matter that much whenever memory is
> > > > mapped above or below 4g.  And for old 32bit guests there is always the
> > > > option to stick with piix which continues to offers up to 3.5G lowmem.
> > > > 
> > > > cheers,
> > > >   Gerd
> > > > 
> > > 
> > > Any update here?
> > 
> > No time to investigate yet, still playing catch-up after xmas holidays &
> > being sick.
> > 
> > > I'm worried 2G lowmem is a bit too aggressive, PAE still exists.
> > 
> > piix (with up to 3.5g lowmem) exists too ;)
> 
> Yes but I think it's preferable to keep features orthogonal.
> 
> > > If we want to support old bios, one way would be to add
> > > a new register to enable 3g lowmem.
> > 
> > Do we really want do this (allow guest change RAM mapping)?
> > 
> > I think we should:
> >  (1) reserve mmconf xbar as motherboard ressource, so it can live within
> >      PCI0._CRS
> 
> I don't think this is possible, PCI FW spec seems to
> outlaw this explicitly.

Nevertheless my laptop does it this way, and it would simplify things
because we'll need a single mmio range below 4k then.

Have a pointer to the spec?

> Right. So you'll look into 4 then? I'm keeping these
> patches out of tree for now ...

Still plan to care when I find time, yes.

cheers,
  Gerd