Re: [PATCH v4 2/4] xen: Add files needed for minimal ppc64le build

[Shawn Anastasio <sanastasio@xxxxxxxxxxxxxxxxxxxxx>]

On 6/16/23 3:27 PM, Andrew Cooper wrote:
> Sorry, I also meant to ask.  How prevalent is Big Endian in practice in
> the Power world?

Modern systems support operating in either endianness, but historically
most operating systems targeted Big Endian-only, and some older systems
didn't support Little Endian at all.

These days Little Endian is more prevalent (at least in the Open Source
world), and many Linux distributions only target LE. Despite this though,
most firmware on Power systems still operates in Big Endian mode
exclusively,
so it's the responsibility of the kernel to handle the switch to LE at
its entrypoint.

The FW being BE also needs to be considered whenever the kernel calls
into firmware, since it's the responsibility of the kernel to switch to BE
before making the call, and also to switch itself back to LE after the FW
routine returns. Typically it's just handled by sprinkling this trampoline
(via a macro) at all of the entry/return points.

> It's another area (like 4k pages) where I expect there to be plenty of
> fun to be had with the codebase.

Hopefully the choice to run in Little Endian mode will reduce the number
of pain points encountered.

Also it's worth mentioning that both 4k and 64k pages are supported,
though 64k is probably the more common choice.

> ~Andrew

Thanks,
Shawn