Re: s390/nospec: add an option to use thunk-extern

Vasily Gorbik
2022-06-27T12:51:14Z

Hi Joe,

sorry for late reply.

> I couldn't find the upstream patch post for 1d2ad084800e ("s390/nospec:
> add an option to use thunk-extern"), so replying off-list here. Feel
> free to cc the appropriate list.
>
> Regarding this change, as I understand it, when CONFIG_EXPOLINE_EXTERN=y
> out-of-tree kernel modules will need to link against
> arch/s390x/lib/expoline.o, right?
>
> And if so, shouldn't the top level 'prepare_modules' target create
> expoline.o for this purpose?

Thanks for bringing this up. I definitely missed out-of-tree kernel modules
build case without a prebuilt kernel. On the other hand this post-linking
trick is a rip off from powerpc:

KBUILD_LDFLAGS_MODULE += arch/powerpc/lib/crtsavres.o

So, now I wonder why powerpc doesn't have crtsavres.o in 'prepare_modules'.

Anyhow, below is couple of patches to consider. The first one is
meant to be backportable, as the second one requires 4efd417f298b.

I had to move expoline.S to a separate directory to be able to call into
its Makefile for 'prepare_modules' and avoid warnings for other targets
defined in the same Makefile. Not sure if there are better kbuild tricks
I could use. Another option I thought about is to keep expoline.S where
it is and add a condition into that Makefile:
expoline_prepare: prepare0
$(Q)$(MAKE) $(build)=arch/s390/lib expoline_prepare=1 arch/s390/lib/expoline.o

arch/s390/lib/Makefile:
# first target defined
obj-$(CONFIG_EXPOLINE_EXTERN) += expoline.o
ifndef expoline_prepare
# ...other targets...

Vasily Gorbik (2):
s390/nospec: build expoline.o for modules_prepare target
s390/nospec: remove unneeded header includes

arch/s390/Makefile | 8 +++++++-
arch/s390/include/asm/nospec-insn.h | 2 --
arch/s390/lib/Makefile | 3 ++-
arch/s390/lib/expoline/Makefile | 3 +++
arch/s390/lib/{ => expoline}/expoline.S | 0
5 files changed, 12 insertions(+), 4 deletions(-)
create mode 100644 arch/s390/lib/expoline/Makefile
rename arch/s390/lib/{ => expoline}/expoline.S (100%)

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2.35.1