[PATCH v3 3/3] powerpc/64s: feature: Work around inline asm issues

[Bill Wendling]

On Thu, Nov 26, 2020 at 5:03 PM Michael Ellerman <mpe at ellerman.id.au> wrote:
>
> Bill Wendling <morbo at google.com> writes:
> > On Mon, Nov 23, 2020 at 7:44 PM Michael Ellerman <mpe at ellerman.id.au> wrote:
> >> Bill Wendling <morbo at google.com> writes:
> >> > On Mon, Nov 23, 2020 at 12:10 PM Segher Boessenkool
> >> > <segher at kernel.crashing.org> wrote:
> >> >> On Mon, Nov 23, 2020 at 12:01:01PM -0800, Bill Wendling wrote:
> >> >> > On Mon, Nov 23, 2020 at 11:58 AM Segher Boessenkool
> >> >> > <segher at kernel.crashing.org> wrote:
> >> >> > > > On Sun, Nov 22, 2020 at 10:36 PM Segher Boessenkool
> >> >> > > > <segher at kernel.crashing.org> wrote:
> >> >> > > > > "true" (as a result of a comparison) in as is -1, not 1.
> >> >> > >
> >> >> > > On Mon, Nov 23, 2020 at 11:43:11AM -0800, Bill Wendling wrote:
> >> >> > > > What Segher said. :-) Also, if you reverse the comparison, you'll get
> >> >> > > > a build error.
> >> >> > >
> >> >> > > But that means your patch is the wrong way around?
> >> >> > >
> >> >> > > -       .ifgt (label##4b- label##3b)-(label##2b- label##1b);    \
> >> >> > > -       .error "Feature section else case larger than body";    \
> >> >> > > -       .endif;                                                 \
> >> >> > > +       .org . - ((label##4b-label##3b) > (label##2b-label##1b)); \
> >> >> > >
> >> >> > > It should be a + in that last line, not a -.
> >> >> >
> >> >> > I said so in a follow up email.
> >> >>
> >> >> Yeah, and that arrived a second after I pressed "send" :-)
> >> >>
> >> > Michael, I apologize for the churn with these patches. I believe the
> >> > policy is to resend the match as "v4", correct?
> >> >
> >> > I ran tests with the change above. It compiled with no error. If I
> >> > switch the labels around to ".org . + ((label##2b-label##1b) >
> >> > (label##4b-label##3b))", then it fails as expected.
> >>
> >> I wanted to retain the nicer error reporting for gcc builds, so I did it
> >> like this:
> >>
> >> diff --git a/arch/powerpc/include/asm/feature-fixups.h b/arch/powerpc/include/asm/feature-fixups.h
> >> index b0af97add751..c4ad33074df5 100644
> >> --- a/arch/powerpc/include/asm/feature-fixups.h
> >> +++ b/arch/powerpc/include/asm/feature-fixups.h
> >> @@ -36,6 +36,24 @@ label##2:                                            \
> >>         .align 2;                                       \
> >>  label##3:
> >>
> >> +
> >> +#ifndef CONFIG_CC_IS_CLANG
> >> +#define CHECK_ALT_SIZE(else_size, body_size)                   \
> >> +       .ifgt (else_size) - (body_size);                        \
> >> +       .error "Feature section else case larger than body";    \
> >> +       .endif;
> >> +#else
> >> +/*
> >> + * If we use the ifgt syntax above, clang's assembler complains about the
> >> + * expression being non-absolute when the code appears in an inline assembly
> >> + * statement.
> >> + * As a workaround use an .org directive that has no effect if the else case
> >> + * instructions are smaller than the body, but fails otherwise.
> >> + */
> >> +#define CHECK_ALT_SIZE(else_size, body_size)                   \
> >> +       .org . + ((else_size) > (body_size));
> >> +#endif
> >> +
> >>  #define MAKE_FTR_SECTION_ENTRY(msk, val, label, sect)          \
> >>  label##4:                                                      \
> >>         .popsection;                                            \
> >> @@ -48,9 +66,7 @@ label##5:                                                     \
> >>         FTR_ENTRY_OFFSET label##2b-label##5b;                   \
> >>         FTR_ENTRY_OFFSET label##3b-label##5b;                   \
> >>         FTR_ENTRY_OFFSET label##4b-label##5b;                   \
> >> -       .ifgt (label##4b- label##3b)-(label##2b- label##1b);    \
> >> -       .error "Feature section else case larger than body";    \
> >> -       .endif;                                                 \
> >> +       CHECK_ALT_SIZE((label##4b-label##3b), (label##2b-label##1b)); \
> >>         .popsection;
> >>
> >>
> >>
> >> I've pushed a branch with all your patches applied to:
> >>
> >>   https://github.com/linuxppc/linux/commits/next-test
> >>
> > This works for me. Thanks!
>
> Great.
>
> >> Are you able to give that a quick test? It builds clean with clang for
> >> me, but we must be using different versions of clang because my branch
> >> already builds clean for me even without your patches.
> >>
> > You may need to set LLVM_IAS=1 to get the behavior I'm seeing. That
> > turns on clang's integrated assembler, which I think is disabled by
> > default.
>
> Yep that does it.
>
> But then I get:
>   clang: error: unsupported argument '-mpower4' to option 'Wa,'
>   clang: error: unsupported argument '-many' to option 'Wa,'
>
> So I guess I'm still missing something?
>
[Resent, because my previous email went out as non-plain text.]

I've seen that too. I'm not entirely sure what's causing it, but I'll
look into it. I've got a backlog of things to work on still. :-) It's
probably a clang issue. There's another one that came up having to do
with the format of some PPC instructions. I have a clang fix on review
for those.

> > Note that with clang's integrated assembler, arch/powerpc/boot/util.S
> > fails to compile. Alan Modra mentioned that he sent you a patch to
> > "modernize" the file so that clang can compile it.
>
> Ah you're right he did, it didn't go to patchwork so I missed it. Have
> grabbed it now.
>
Thanks!

-bw