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

[Michael S. Tsirkin]

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.

>  (2) make PCI0._CRS start at end of lowmem, like it does on piix

This we can do. But we'll have to make a hole for the xbar.

> Then maybe:
>  (3) update seabios to place xbar somewhere else
>  (4) adjust memory layout.
> 
> (1)+(2) are useful anyway.  (3)+(4) would allow for 3g lowmem, and
> obviously have some compatibility issues:  doing (4) requires a seabios
> update.
> 
> cheers,
>   Gerd

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