Re: [PATCH bpf v3] x86/cpufeature: bpf hack for clang not supporting asm goto

Thomas Gleixner
2018-05-13T18:02:35Z

On Sun, 13 May 2018, Alexei Starovoitov wrote:
> On Sat, May 12, 2018 at 10:30:02PM +0200, Thomas Gleixner wrote:
> > But yes, the situation is slightly different here because tools which
> > create trace event magic _HAVE_ to pull in kernel headers. At the same time
> > these tools depend on a compiler which failed to implement asm_goto for
> > fricking 8 years.
>
> As a maintainer of a piece of llvm codebase I have to say that
> this bullying tactic has the opposite effect.

I'm not bullying at all. Its a fact that the discussion about asm goto is
dragging out 8 years now. We've stayed away from mandating it for quite
some time, but at some point it just doesn't make sense anymore.

> The inline asm is processed by gcc and llvm very differently. gcc is
> leaking internal backend implementation details into inline asm
> syntax. It makes little sense for llvm to do the same, since compiler
> codegen is completely different. gcc doesn't have integrated assembler
> whereas llvm not only can parse asm, but can potentially optimize it as
> well. Instead of demanding asm-goto that matches gcc one to one it's
> better to work with the community to define the syntax that works for
> both kernel and llvm.

Come on, we surely are open for discussions, but what I've seen so far is
just 'oh we can't do this because' instead of a sane proposal how it can be
done w/o rewriting the whole ASM GOTO stuff in the kernel or even
duplicating it.

> > + * Workaround for the sake of BPF compilation which utilizes kernel
> > + * headers, but clang does not support ASM GOTO and fails the build.
> > + */
> > +#ifndef __BPF__
> > +#warning "Compiler lacks ASM_GOTO support. Add -D __BPF__ to your compiler arguments"
> > +#endif
>
> Agree.
> The warning makes sense to me, but it has to be different macro name.
> How about -D__BPF_TRACING__ or -D__BPF_KPROBES__ or something similar ?

Fair enough.

> Such name will also make it clear that only tracing bpf programs
> need this. Networking programs shouldn't be including kernel headers.
> There was never a need, but since the tracing progs are often used
> as an example people copy paste makefiles too.
> We tried to document it as much as possible, but people still use
> 'clang -target native -I/kernel/includes bpf_prog.c -emit-llvm | llc -march=bpf'
> in their builds.
> (sometimes as a workaround for setups where clang is older version,
> but llc/llvm is new)
> Now they will see this warning and it will force them to think whether
> they actually need the kernel headers.

Makes sense.

> > +
> > +#define static_cpu_has(bit) boot_cpu_has(bit)
> > +
> > +#else
> > +
> > /*
> > * Static testing of CPU features. Used the same as boot_cpu_has().
> > * These will statically patch the target code for additional
> > @@ -195,6 +209,7 @@ static __always_inline __pure bool _stat
> > boot_cpu_has(bit) : \
> > _static_cpu_has(bit) \
> > )
> > +#endif
> >
> > #define cpu_has_bug(c, bit) cpu_has(c, (bit))
> > #define set_cpu_bug(c, bit) set_cpu_cap(c, (bit))
> > --- a/samples/bpf/Makefile
> > +++ b/samples/bpf/Makefile
> > @@ -255,7 +255,7 @@ verify_target_bpf: verify_cmds
> > $(obj)/%.o: $(src)/%.c
> > $(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) -I$(obj) \
> > -I$(srctree)/tools/testing/selftests/bpf/ \
> > - -D__KERNEL__ -Wno-unused-value -Wno-pointer-sign \
> > + -D__KERNEL__ -D__BPF__ -Wno-unused-value -Wno-pointer-sign \
>
> Yep. In samples/bpf and libbcc we can selectively add -D__BPF_TRACING__
> I think sysdig and other folks can live with that as well.
> Agree?

Sure. Care to send an updated patch?

Thanks,

tglx