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

[Joe Lawrence <joe.lawrence@redhat.com>]

On 6/27/22 8:50 AM, Vasily Gorbik wrote:
> 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%)
> 

Thanks, Vasily.  We'll test these with OOT and the original gitlab
pipeline where we spotted potential issue with packaging and report back.

-- 
Joe