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Date:   Wed, 16 Sep 2020 13:34:49 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Rong Chen <rong.a.chen@intel.com>,
        kernel test robot <lkp@intel.com>,
        "Li, Philip" <philip.li@intel.com>, x86-ml <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        clang-built-linux <clang-built-linux@googlegroups.com>,
        Marco Elver <elver@google.com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING
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On Tue, Sep 15, 2020 at 05:50:54PM -0400, Arvind Sankar wrote:
> On Tue, Sep 15, 2020 at 10:49:12PM +0200, Borislav Petkov wrote:
> > 
> > (Btw, clang doesn't need to add that "xor %eax,%eax" - panic() should not be
> >  returning, ever. :-))
> > 
> 
> I think this is because panic() is varargs, and clang doesn't support
> gcc's -mskip-rax-setup. The normal ABI requires the caller to set RAX to
> the number of arguments in vector registers.
> 
> https://patchwork.ozlabs.org/project/gcc/patch/20141218131150.GA32638@intel.com/

Ah, good point. Found this in the ABI doc:

"For calls that may call functions that use varargs or stdargs
(prototype-less calls or calls to functions containing ellipsis (...) in
the declaration) %al is used as hidden argument to specify the number of
vector registers used. The contents of %al do not need to match exactly
the number of registers, but must be an upper bound on the number of
vector registers used and is in the range 0–8 inclusive."

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette