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

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Date:   Tue, 6 Feb 2018 14:48:45 -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

El Tue, Feb 06, 2018 at 02:37:06PM -0800 Greg Kroah-Hartman ha dit:

> On Tue, Feb 06, 2018 at 01:59:41PM -0800, Matthias Kaehlcke wrote:
> > 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.
> 
> That patch doesn't apply cleanly to the 4.4.y or 4.9.y trees anymore.
> It seems that only one hunk of it is really needed, the #ifndef change,
> right?  If so, I'll be glad to apply that portion.

Indeed, only the #ifndef change is needed.

Thanks!

m.