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

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

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>

-- 
Joe