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Date:   Wed, 16 Sep 2020 13:34:49 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Rong Chen <rong.a.chen@...el.com>,
        kernel test robot <lkp@...el.com>,
        "Li, Philip" <philip.li@...el.com>, x86-ml <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Marco Elver <elver@...gle.com>
Subject: Re: [tip:x86/seves] BUILD SUCCESS WITH WARNING
 e6eb15c9ba3165698488ae5c34920eea20eaa38e

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.

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