linux-kernel - Re: perf test LLVM & clang 6 failing

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Date:   Tue, 28 Nov 2017 14:55:53 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>, Yonghong Song <yhs@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Wang Nan <wangnan0@...wei.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        David Ahern <dsahern@...il.com>, Jiri Olsa <jolsa@...nel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>
Subject: Re: perf test LLVM & clang 6 failing

Em Mon, Nov 27, 2017 at 01:45:21PM -0800, Matthias Kaehlcke escreveu:
> El Mon, Nov 27, 2017 at 01:57:56PM -0600 Josh Poimboeuf ha dit:
> 
> > On Mon, Nov 27, 2017 at 04:34:25PM -0300, Arnaldo Carvalho de Melo wrote:
> > > Em Mon, Nov 27, 2017 at 11:11:56AM -0800, Yonghong Song escreveu:
> > > > On 11/27/17 9:04 AM, Arnaldo Carvalho de Melo wrote:
> > > > > Em Fri, Nov 24, 2017 at 04:16:52PM +0100, Daniel Borkmann escreveu:
> > > > > > [ +Yonghong ]
> > > > > 
> > > > > + Josh
> > > > > > On 11/24/2017 03:47 PM, Arnaldo Carvalho de Melo wrote:
> > > > > > > FYI, just noticed, recently updated clang to version 6, from its
> > > > > > > upstream git repo.
> > > > > > 
> > > > > > Do you recall what was your LLVM version prior to this where it was
> > > > > > working fine? (Wild guess from below would be the BPF inline asm
> > > > > > support that was added recently to LLVM (2865ab6996) vs asm() used
> > > > > > in headers included in the stdin header causing trouble due to arch
> > > > > > mixup?)
> > > > > 
> > > > > So, if I go to the cset just before:
> > > > > 
> > > > > commit f5caf621ee357279e759c0911daf6d55c7d36f03
> > > > > Author: Josh Poimboeuf <jpoimboe@...hat.com>
> > > > > Date:   Wed Sep 20 16:24:33 2017 -0500
> > > > > 
> > > > >      x86/asm: Fix inline asm call constraints for Clang
> > > > > ---
> > > > > 
> > > > > 'perf test LLVM' works again:
> > > > > 
> > > > > [root@...et ~]# perf test LLVM
> > > > > 37: LLVM search and compile                               :
> > > > > 37.1: Basic BPF llvm compile                              : Ok
> > > > > 37.2: kbuild searching                                    : Ok
> > > > > 37.3: Compile source for BPF prologue generation          : Ok
> > > > > 37.4: Compile source for BPF relocation                   : Ok
> > > > > [root@...et ~]#
> > > > > 
> > > > > I.e. 'perf test LLVM' built from what is in my acme/perf/urgent branch,
> > > > > targetted to v4.15, uses kernel headers and if I go to just before
> > > > > f5caf621ee, it works again, both with clang from fedora26 (4.0.1) and
> > > > > with 6.0, built from sources.
> > > > 
> > > > This patch introduced a module level inline assembly.
> > > > 
> > > > ...
> > > > --- a/arch/x86/include/asm/asm.h
> > > > +++ b/arch/x86/include/asm/asm.h
> > > > @@ -132,4 +132,15 @@
> > > >  /* For C file, we already have NOKPROBE_SYMBOL macro */
> > > >  #endif
> > > > 
> > > > +#ifndef __ASSEMBLY__
> > > > +/*
> > > > + * This output constraint should be used for any inline asm which has a
> > > > "call"
> > > > + * instruction.  Otherwise the asm may be inserted before the frame pointer
> > > > + * gets set up by the containing function.  If you forget to do this,
> > > > objtool
> > > > + * may print a "call without frame pointer save/setup" warning.
> > > > + */
> > > > +register unsigned int __asm_call_sp asm("esp");
> > > > +#define ASM_CALL_CONSTRAINT "+r" (__asm_call_sp)
> > > > +#endif
> > > > ...
> > > > 
> > > > This will cause "clang ... -target bpf ..." failure since 4.0 does not have
> > > > bpf inline asm support and 6.0 does not recognize the register 'esp'.
> > > 
> > > Ok, explains the problem then, Josh, ideas on how to proceed here?
> > 
> > The original change to add the global inline asm:
> > 
> >   5caf621ee35 ("x86/asm: Fix inline asm call constraints for Clang")
> > 
> > was done to support clang in the first place.  Before that change, a
> > clang-built kernel didn't even boot.  So I'm a bit perplexed by the fact
> > that this change would be causing clang problems, since it was done to
> > fix clang in the first place.
> > 
> > Adding Andrey and Matthias, maybe they can help clarify things.
> 
> Indeed the change was needed to boot on x86.
> 
> I know next to nothing about BPF, if I understand correctly the error
> is generated when compiling for the BPF "architecture" not for x86. In
> this process x86 assembly headers are included, one of which contains
> the declaration of the register variable, in an register that exists
> on x86, but not BPS.
> 
> I guess the first questions is whether the x86 asm headers should/need
> to be included when compiling for BPF. If this needed/can not be
> easily avoided one option could be to put the declaration within an
> ifdef __x86_64__ block.

Right, unsure if this is the way to go, but fixes the problem here:

diff --git a/arch/x86/include/asm/asm.h b/arch/x86/include/asm/asm.h
index 219faaec51df..d180d573fd38 100644
--- a/arch/x86/include/asm/asm.h
+++ b/arch/x86/include/asm/asm.h
@@ -136,6 +136,8 @@
 #endif
 
 #ifndef __ASSEMBLY__
+#if defined(__i386__) || defined(__x86_64__)
+/* bpf target builds also include these headers */
 /*
  * This output constraint should be used for any inline asm which has a "call"
  * instruction.  Otherwise the asm may be inserted before the frame pointer
@@ -145,5 +147,6 @@
 register unsigned long current_stack_pointer asm(_ASM_SP);
 #define ASM_CALL_CONSTRAINT "+r" (current_stack_pointer)
 #endif
+#endif
 
 #endif /* _ASM_X86_ASM_H */

[root@...et ~]# perf test LLVM
37: LLVM search and compile                               :
37.1: Basic BPF llvm compile                              : Ok
37.2: kbuild searching                                    : Ok
37.3: Compile source for BPF prologue generation          : Ok
37.4: Compile source for BPF relocation                   : Ok
[root@...et ~]#

Is there any define clang generates for the bpf target?

- Arnaldo