Re: [Qemu-devel] [PATCH] q35: split memory at 2G

[Gerd Hoffmann]

On Tue, May 28, 2019 at 10:49:55PM -0400, Michael S. Tsirkin wrote:
> On Wed, May 29, 2019 at 03:21:16AM +0200, Paolo Bonzini wrote:
> > On 28/05/19 22:48, Gerd Hoffmann wrote:
> > > Original q35 behavior was to split memory 2.75 GB, leaving space for the
> > > mmconfig bar at 0xb000000 and pci I/O window starting at 0xc0000000.
> > > 
> > > Note: Those machine types have been removed from the qemu codebase
> > > meanwhile because they could not be live-migrated so there was little
> > > value in keeping them around.
> > > 
> > > With the effort to allow for gigabyte-alignment of guest memory that
> > > behavior was changed:  The split was moved to 2G, but only in case the
> > > memory didn't fit below 2.75 GB.
> > > 
> > > So today the address space between 2G and 2,75G is not used for guest
> > > memory in typical use cases, where the guest memory sized at a power of
> > > two or a gigabyte number.  But if you configure your guest with some odd
> > > amout of memory (such as 2.5G) the address space is used.
> > 
> > Wasn't it done to ensure pre-PAE OSes could use as much memory as
> > possible?  (If you run pre-PAE OSes with more RAM than can fit below 4G,
> > you can just reduce the amount of memory and get all the 2.75G).
> > 
> > Paolo
> 
> Absolutely. Gerd is just saying the configuration is rare enough that
> it's not worth worrying about. I don't know myself - why do
> we bother making this change? What's the advantage?

Some ovmf versions place the mmconfig @ 2G.  Which works fine in 99% of
the cases, but with memory sizes between 2G and 2.75G it doesn't.

cheers,
  Gerd