linux-kernel - Re: [PATCH] x86: use register variable to get stack pointer value

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Date:   Fri, 29 Sep 2017 14:16:07 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: use register variable to get stack pointer value

On Fri, Sep 29, 2017 at 11:37:52AM -0700, Andy Lutomirski wrote:
> On Fri, Sep 29, 2017 at 8:38 AM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> > On Fri, Sep 29, 2017 at 05:15:36PM +0300, Andrey Ryabinin wrote:
> >> Currently we use current_stack_pointer() function to get the value
> >> of the stack pointer register. Since commit f5caf621ee35
> >> ("x86/asm: Fix inline asm call constraints for Clang") we have stack
> >> register variable declared. It can be used instead of current_stack_pointer()
> >> function which allows to optimize away some excessive "mov %rsp, %<dst>"
> >> instructions:
> >>
> >> -mov    %rsp,%rdx
> >> -sub    %rdx,%rax
> >> -cmp    $0x3fff,%rax
> >> -ja     ffffffff810722fd <ist_begin_non_atomic+0x2d>
> >>
> >> +sub    %rsp,%rax
> >> +cmp    $0x3fff,%rax
> >> +ja     ffffffff810722fa <ist_begin_non_atomic+0x2a>
> >>
> >> Remove current_stack_pointer(), rename __asm_call_sp to current_stack_pointer
> >> and use it instead of removed function.
> >>
> >> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> >
> > Reviewed-by: Josh Poimboeuf <jpoimboe@...hat.com>
> 
> 
> Ok with me.  As an alternative, you could leave it as
> current_stack_pointer(), but either way is fine.

Good point, I think changing current_stack_pointer() to return the
global variable would be cleaner.

-- 
Josh