linux-kernel - Re: [PATCH 4.4 05/53] x86/asm: Use register variable to get stack pointer value

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Message-ID: <20180206215941.GA99249@google.com>
Date:   Tue, 6 Feb 2018 13:59:41 -0800
From:   Matthias Kaehlcke <mka@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        David Woodhouse <dwmw@...zon.co.ku>,
        Razvan Ghitulete <rga@...zon.de>,
        Guenter Roeck <groeck@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Greg Hackmann <ghackmann@...gle.com>
Subject: Re: [PATCH 4.4 05/53] x86/asm: Use register variable to get stack
 pointer value

Hi Greg,

El Mon, Jan 22, 2018 at 09:39:57AM +0100 Greg Kroah-Hartman ha dit:

> 4.4-stable review patch.  If anyone has any objections, please let me know.
> 
> ------------------
> 
> From: Andrey Ryabinin <aryabinin@...tuozzo.com>
> 
> commit 196bd485ee4f03ce4c690bfcf38138abfcd0a4bc upstream.
> 
> 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 a 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 the removed function.
> 
> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
> Reviewed-by: Josh Poimboeuf <jpoimboe@...hat.com>
> Cc: Andy Lutomirski <luto@...nel.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Link: http://lkml.kernel.org/r/20170929141537.29167-1-aryabinin@virtuozzo.com
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> [dwmw2: We want ASM_CALL_CONSTRAINT for retpoline]
> Signed-off-by: David Woodhouse <dwmw@...zon.co.ku>
> Signed-off-by: Razvan Ghitulete <rga@...zon.de>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

We recently merged this patch to the Chrome OS kernel tree and it
broke our x86 builds with clang:

arch/x86/include/asm/asm.h:116:50: error: register 'rsp' unsuitable for global register variables on this target
register unsigned long current_stack_pointer asm(_ASM_SP);
                                                 ^
arch/x86/include/asm/asm.h:41:18: note: expanded from macro '_ASM_SP'
#define _ASM_SP         __ASM_REG(sp)
                        ^
arch/x86/include/asm/asm.h:24:32: note: expanded from macro '__ASM_REG'
#define __ASM_REG(reg)         __ASM_SEL_RAW(e##reg, r##reg)
                               ^
arch/x86/include/asm/asm.h:19:29: note: expanded from macro '__ASM_SEL_RAW'
# define __ASM_SEL_RAW(a,b) __ASM_FORM_RAW(b)
                            ^
arch/x86/include/asm/asm.h:10:32: note: expanded from macro '__ASM_FORM_RAW'
# define __ASM_FORM_RAW(x)     #x
                               ^
<scratch space>:4:1: note: expanded from here
"rsp"
^
1 error generated.


This can be fixed by also integrating the following patch:

commit 520a13c530aeb5f63e011d668c42db1af19ed349
Author: Josh Poimboeuf <jpoimboe@...hat.com>
Date:   Thu Sep 28 16:58:26 2017 -0500

    x86/asm: Fix inline asm call constraints for GCC 4.4


Admittedly a v4.4 kernel built with clang + LTS merges is a very
special case and we can fix this in Chrome OS by integrating the above
patch locally. Still it would be good to get it into stable to avoid
others from running into this, especially since the fix is very
simple.

Actually I just noticed that the patch also isn't in v4.9, which could
extend the number of affected 'users' significantly, so I think we
almost certainly want Josh's patch in stable.

Thanks

Matthias