Hi,
The change [2] itself is rather old, so I wondered if I'm missing that this was implemented in a totally different way. Do I have to switch/set options these days instead of using that patch?
It should just work. qemu passes ram regions to the firmware using fw_cfg (etc/e820) these days, and both seabios and ovmf support this for years already.
So the question become why the change is not upstream yet? Was it maybe discussed in the past and Nack'ed for some reason?
Exactly.
RHEL/CentOS 6 implemented the extra cmos byte, but this was not accepted upstream in favor of the more flexible e820 solution.
RHEL/CentOS 7 supports both etc/e820 (upstream) and cmos (el6), but the later is only there for backward compatibility reasons.
cheers, Gerd