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

[Michael S. Tsirkin]

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?

-- 
MST