Re: Query regarding hole in EFI Memory Map

["Prakhya, Sai Praneeth" <sai.praneeth.prakhya@intel.com>]

> Of all the gin joints in all the towns in all the world, Prakhya, Sai Praneeth had to
> walk into mine at 16:30 on Monday 14 May 2018 and say:
> 
> > Hi All,
> >
> > Recently, I have observed that there was a hole in EFI Memory Map
> > passed by firmware to Linux kernel. So, wanted to check with you if
> > this is expected or not.
> >
> > My Test setup:
> > I usually boot qemu with OVMF and Linux kernel. I use below command to
> > boot kernel. "qemu-system-x86_64 -cpu host -hda <live-image> -serial
> > stdio -bios <OVMF.fd> -m 2G -enable-kvm -smp 2"
> >
> > I have noticed that the EFI Memory Map (printed by kernel) is almost
> > contiguous but with only one hole ranging from 0xA0000 to 0x100000. As
> > far as I know, kernel hasn't modified this EFI Memory Map, so I am
> > assuming that firmware has passed memory map with a hole. I have
> > looked at UEFI spec "GetMemoryMap()" definition, and it says "The map
> > describes all of memory, no matter how it is being used". So, I am
> > thinking that EFI Memory Map shouldn't have any holes, am I correct?
> > If not, could someone please explain me the reason for this hole in EFI
> Memory Map.
> 
> The map may describe all of physical RAM, however it is not necessarily the case
> that all available RAM be physically contiguous.
> 
> With older IBM PCs based on the Intel 8088 processor, you could only have a
> 1MB address space. The first 640KB was available for RAM. The remaining space
> traditionally contained memory-mapped option ROMs, particularly for things
> like the video BIOS routines. The VGA text screen was also mapped to 0xB8000.
> 
> Obviously, later processors made it possible to have additional memory above
> 1MB (sometimes called "high memory"), but for backward compatibility
> purposes, the gap from 0xA0000 to 0xFFFFF remained.
> 
> So basically, on Intel machines you will always see this gap in RAM due to
> "hysterical raisins." It's just an artifact of the platform design. (And for that
> reason you'll see it both with the UEFI memory map facility and the legacy E820
> BIOS facility).

Thanks a lot! for the explanation Bill. I really appreciate it :)

Regards,
Sai