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

[Sumanth Korikkar <sumanthk@linux.ibm.com>]

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