Re: [PATCH] x86/build: fix compiler support check for CONFIG_RETPOLINE

[Masahiro Yamada]

On Wed, Dec 5, 2018 at 12:23 PM Zhenzhong Duan
<zhenzhong.duan@xxxxxxxxxx> wrote:
>
> Hi Masahiro,
>
>
> Thanks for fixing.
> There is a question still confusing me. There are many CONFIG_* used in
> arch/x86/Makefile.
> Will they also be impacted or not?


The top level Makefile (and arch/*/Makefile) is getting huge.
Parsing the whole Makefile everytime means,
it computes variables that may or may not be used.

$(call cc-option,...) is a bit costly,
but other parts are unnoticeable
in terms of performance.


I am addressing the use of $(error ...) / $(warning ...)
in the non-recipe context since they could break the build
for false positive reasons.




> # grep CONFIG_ arch/x86/Makefile
> ifeq ($(CONFIG_X86_32),y)
>          cflags-$(CONFIG_MK8) += $(call cc-option,-march=k8)
>          cflags-$(CONFIG_MPSC) += $(call cc-option,-march=nocona)
>   ......
> drivers-$(CONFIG_MATH_EMULATION) += arch/x86/math-emu/
> drivers-$(CONFIG_PCI)      += arch/x86/pci/
> drivers-$(CONFIG_OPROFILE) += arch/x86/oprofile/
> drivers-$(CONFIG_PM) += arch/x86/power/
> drivers-$(CONFIG_FB) += arch/x86/video/
> ifeq ($(CONFIG_X86_DECODER_SELFTEST),y)
>
> Thanks
> Zhenzhong
> On 2018/12/5 11:00, Masahiro Yamada wrote:
> > It is wrong to add CONFIG option diagnostic to the Makefile parse
> > stage.
> >
> > Once you are hit by the error about non-retpoline compiler, the
> > compilation still breaks even after disabling CONFIG_RETPOLINE.
> >
> > The easiest fix is to move this check to the "archprepare" like commit
> > 829fe4aa9ac1 ("x86: Allow generating user-space headers without a
> > compiler") did.
> >
> > Link: https://lkml.org/lkml/2018/12/4/206
> > Reported-by: Meelis Roos <mroos@xxxxxxxx>
> > Fixes: 4cd24de3a098 ("x86/retpoline: Make CONFIG_RETPOLINE depend on compiler support")
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
> > ---
> >
> >   arch/x86/Makefile | 14 ++++++++------
> >   1 file changed, 8 insertions(+), 6 deletions(-)
> >
> > diff --git a/arch/x86/Makefile b/arch/x86/Makefile
> > index f5d7f41..ae0148b 100644
> > --- a/arch/x86/Makefile
> > +++ b/arch/x86/Makefile
> > @@ -219,12 +219,7 @@ KBUILD_CFLAGS += -Wno-sign-compare
> >   KBUILD_CFLAGS += -fno-asynchronous-unwind-tables
> >
> >   # Avoid indirect branches in kernel to deal with Spectre
> > -ifdef CONFIG_RETPOLINE
> > -ifeq ($(RETPOLINE_CFLAGS),)
> > -  $(error You are building kernel with non-retpoline compiler, please update your compiler.)
> > -endif
> > -  KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
> > -endif
> > +KBUILD_CFLAGS += $(RETPOLINE_CFLAGS)
> >
> >   archscripts: scripts_basic
> >       $(Q)$(MAKE) $(build)=arch/x86/tools relocs
> > @@ -307,6 +302,13 @@ ifndef CC_HAVE_ASM_GOTO
> >       @echo Compiler lacks asm-goto support.
> >       @exit 1
> >   endif
> > +ifdef CONFIG_RETPOLINE
> > +ifeq ($(RETPOLINE_CFLAGS),)
> > +     @echo "You are building kernel with non-retpoline compiler." >&2
> > +     @echo "Please update your compiler." >&2
> > +     @false
> > +endif
> > +endif
> >
> >   archclean:
> >       $(Q)rm -rf $(objtree)/arch/i386



--
Best Regards
Masahiro Yamada