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

[Paolo Bonzini <pbonzini@redhat.com>]

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