Christophe Leroy [off-list ref] writes:

Le 05/10/2022 à 07:32, Benjamin Gray a écrit :

quoted

Implement static call support for 64 bit V2 ABI. This requires making
sure the TOC is kept correct across kernel-module boundaries. As a
secondary concern, it tries to use the local entry point of a target
wherever possible. It does so by checking if both tramp & target are
kernel code, and falls back to detecting the common global entry point
patterns if modules are involved. Detecting the global entry point is
also required for setting the local entry point as the trampoline
target: if we cannot detect the local entry point, then we need to
convservatively initialise r12 and use the global entry point.

The trampolines are marked with `.localentry NAME, 1` to make the
linker save and restore the TOC on each call to the trampoline. This
allows the trampoline to safely target functions with different TOC
values.

However this directive also implies the TOC is not initialised on entry
to the trampoline. The kernel TOC is easily found in the PACA, but not
an arbitrary module TOC. Therefore the trampoline implementation depends
on whether it's in the kernel or not. If in the kernel, we initialise
the TOC using the PACA. If in a module, we have to initialise the TOC
with zero context, so it's quite expensive.

Signed-off-by: Benjamin Gray <redacted>

This looks good to me

Reviewed-by: Christophe Leroy <redacted>

However, thinking out loudly, I'm wondering, could we make things any 
simpler when CONFIG_MODULES is not selected, or is that a too much 
corner case on PPC64 ?

I'd say it's mostly a corner case.

Obviously no distros ship with modules disabled. 

AFAIK even the stripped down kernels we use in CPU bringup have modules
enabled.

So I think it's probably not worth worrying about, unless there's an
obvious and fairly simple optimisation.

cheers