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Date: Thu, 20 Jul 2017 10:18:13 -0500
From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Matthias Kaehlcke <mka@chromium.org>,
        Chris J Arges <chris.j.arges@canonical.com>,
        Borislav Petkov <bp@suse.de>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
        "x86@kernel.org" <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        Douglas Anderson <dianders@chromium.org>,
        Michael Davidson <md@google.com>, Greg Hackmann <ghackmann@google.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Stephen Hines <srhines@google.com>, Kees Cook <keescook@chromium.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Bernhard =?utf-8?Q?Rosenkr=C3=A4nzer?= 
        <Bernhard.Rosenkranzer@linaro.org>
Subject: Re: [PATCH] Revert "x86/uaccess: Add stack frame output operand in
 get_user() inline asm"
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On Thu, Jul 20, 2017 at 01:01:39PM +0300, Andrey Ryabinin wrote:
> 2017-07-19 20:46 GMT+03:00 Josh Poimboeuf <jpoimboe@redhat.com>:
> 
> >
> > After doing some testing, I don't think this approach is going to work
> > after all.  In addition to forcing the stack frame, it also causes GCC
> > to add an unnecessary extra instruction to the epilogue of each affected
> > function:
> >
> >   lea    -0x10(%rbp),%rsp
> >
> > We shouldn't be inserting extra instructions like that.  I also don't
> > think the other suggestion of turning the '__sp' register variable into
> > a global variable is a very good solution either, as that just wastes
> > memory for no reason.
> >
> 
> Wastes memory? How is that wastes memory? That doesn't make any sense.

Yes, you're right, that doesn't make any sense.  I think I was trying to
wrap my head around what it means to have a global register variable --
in a header file no less -- and why clang would treat that differently
than a local register variable.

-- 
Josh