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

Sumanth Korikkar
2022-07-17T13:11:29Z

Hi Joe,

c4e789572557 ("s390/nospec: build expoline.o for modules_prepare
target") is now in linux.git.

Note: arch/s390/lib/expoline.o is moved to
arch/s390/lib/expoline/expoline.o. This means kernel-devel package in
fedora should also include this updated file path.

Thanks

On 7/1/22 23:39, Joe Lawrence wrote:
> On 7/1/22 5:18 PM, C. Erastus Toe wrote:
>> On Wed, Jun 29, 2022 at 11:16 AM Joe Lawrence <joe.lawrence@redhat.com
>> <mailto:joe.lawrence@redhat.com>> wrote:
>>
>> 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.
>>
>> Hi,
>>
>> Successfully tested the first patch in a rhel-9 backport. (had to skip
>> the second as it has dependencies on other patches like [1] that
>> deprecated symbols like __LC_BR_R1. Without those, the build resulted in
>> a flood of: depmod: WARNING: <module>.ko needs unknown symbol __LC_BR_R1.)
>>
>> For ("s390/nospec: build expoline.o for modules_prepare target"),
>> Tested-by: C. Erastus Toe <ctoe@redhat.com <mailto:ctoe@redhat.com>>
>>
>> [1] 4efd417f298b ("s390: raise minimum supported machine generation to z10")
>>
>
> And then for the entire series (tested on top of v5.19-rc4),
> Tested-by: Joe Lawrence <joe.lawrence@redhat.com>
>

--
Sumanth