Re: [PATCH] x86/boot: define CC_HAVE_ASM_GOTO

[Nick Desaulniers]

On Tue, Sep 25, 2018 at 12:08 PM Borislav Petkov <bp@xxxxxxx> wrote:
>
> On Tue, Sep 25, 2018 at 11:41:19AM -0700, Nick Desaulniers wrote:
> > bumping for review.
> >
> > On Fri, Sep 7, 2018 at 1:26 PM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote:
> > >
> > > Since this file steamrolls KBUILD_CFLAGS, we have to redefine these
> > > symbols.
>
> Why do we have to redefine these symbols?
>
> I don't see arch/x86/boot/ using asm_volatile_goto() and
> CC_HAVE_ASM_GOTO anywhere.

They may not explicitly invoke asm_volatile_goto(), but many
translation units under arch/x86/boot/compressed/ share this include
path, triggering this warning:

In file included from arch/x86/boot/compressed/misc.h:20:
In file included from ./include/linux/elf.h:5:
In file included from ./arch/x86/include/asm/elf.h:8:
In file included from ./include/linux/thread_info.h:38:
In file included from ./arch/x86/include/asm/thread_info.h:53:
./arch/x86/include/asm/cpufeature.h:150:2: warning: "Compiler lacks
ASM_GOTO support. Add -D __BPF_TRACING__ to your compiler arguments"
[-W#warnings]
#warning "Compiler lacks ASM_GOTO support. Add -D __BPF_TRACING__ to
your compiler arguments"
 ^

specifically 6 instances from:
In file included from arch/x86/boot/compressed/early_serial_console.c:1:
In file included from arch/x86/boot/compressed/cmdline.c:2:
In file included from arch/x86/boot/compressed/error.c:7:
In file included from arch/x86/boot/compressed/kaslr.c:29:
In file included from arch/x86/boot/compressed/kaslr_64.c:22:
In file included from arch/x86/boot/compressed/misc.c:15:

Note that the check in arch/x86/include/asm/cpufeature.h has the check:

143 #if defined(__clang__) && !defined(CC_HAVE_ASM_GOTO)

so this is hidden from GCC.  I can include this information in the
commit message for a v2 if you'd like?

>
> > > This will prevent warnings in source files in this directory
> > > when Clang supports asm goto.
>
> This statement I can't grok either. Maybe I'm slow or maybe I need more
> background info...

We're in the process of adding asm goto support to Clang.  I'm helping
test by working through compiling the Linux kernel with Clang for
x86_64.  The patch set still has some kinks we're working out, but
having this fix in place will make for smoother sailing once we're
good to go, and we would appreciate having it.

-- 
Thanks,
~Nick Desaulniers