linux-kernel - Re: 2.6.23-rc3-mm1

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Message-ID: <20070823122825.GA29724@uranus.ravnborg.org>
Date:	Thu, 23 Aug 2007 14:28:25 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Andy Whitcroft <apw@...dowen.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...e.de>
Subject: Re: 2.6.23-rc3-mm1

On Thu, Aug 23, 2007 at 01:03:18PM +0100, Andy Whitcroft wrote:
> On Wed, Aug 22, 2007 at 11:10:29AM -0700, Andrew Morton wrote:
> [...]
> > >   CC      arch/x86_64/kernel/asm-offsets.s
> > > arch/x86_64/kernel/asm-offsets.c:1: error: -mpreferred-stack-boundary=3
> > > is not between 4 and 12
> > > make[1]: *** [arch/x86_64/kernel/asm-offsets.s] Error 1
> [...]
> > x86_64-mm-less-stack-alignment.patch has
> > 
> > cflags-y += $(call cc-option,-mpreferred-stack-boundary=3)
> > 
> > So we _should_ have detected that gcc didn't like =3, so it
> > should not have been used.
> > 
> > I am suspecting a kbuild glitch: asm-offsets.c tends to be handled
> > in special ways (ie: it's usually the thing which blows up first)
> > so perhaps it is somehow avoiding the above does-gcc-support-this test.
> > 
> > Suitable cc's added ;)
> 
> It seems that this is a problem caused by the way we check for
> compiler options in x86_64.  Each compiler flag is checked for
> individually and if available added to cflags-y, later that is
> added to CFLAGS.  However, this means that each flag is checked
> in total isolation.  On x86_64 (on this compiler at least) the
> -mpreferred-stack-boundary and -m{32,64} flags are actually mutually
> dependant, the alignment constraints vary based on the word size.
> This leads to the compile failure:
> 
>     # gcc -mpreferred-stack-boundary=3 -S -xc /dev/null  -o FOO
>     # echo $?
>     0
>     # gcc -m64 -mpreferred-stack-boundary=3 -S -xc /dev/null  -o FOO
>     /dev/null:1: error: -mpreferred-stack-boundary=3 is not between 4 and 12
>     # echo $?
>     1

Thats makes sense - good catch.

> 
> In the main Makefile we always add each flag directly to CFLAGS
> which means we check them all in combination, perhaps this is
> prudent here also?  Either way I suspect that changing the -m64
> check to add itself directly to CFLAGS will fix this us.

Something like this then:
[PATCH] x86_64: fix preferred-stack-boundary check

gcc has different interpretation of the -preferred-stack-boundary flag
dependent on the option -m64 is present or not as seen in the following:
     # gcc -mpreferred-stack-boundary=3 -S -xc /dev/null  -o FOO
     # echo $?
     0
     # gcc -m64 -mpreferred-stack-boundary=3 -S -xc /dev/null  -o FOO
     /dev/null:1: error: -mpreferred-stack-boundary=3 is not between 4 and 12
     # echo $?
     1

Adding the -m64 to CFLAGS let cc-option do the right thing.

Thanks to Andy Whitcroft <apw@...dowen.org> for spotting the root cause.

Cc: Andy Whitcroft <apw@...dowen.org> 
Signed-off-by: Sam Ravnborg <sam@...nborg.org>
---
diff --git a/arch/x86_64/Makefile b/arch/x86_64/Makefile
index 128561d..5402c0a 100644
--- a/arch/x86_64/Makefile
+++ b/arch/x86_64/Makefile
@@ -25,6 +25,8 @@ LDFLAGS		:= -m elf_x86_64
 OBJCOPYFLAGS	:= -O binary -R .note -R .comment -S
 LDFLAGS_vmlinux :=
 CHECKFLAGS      += -D__x86_64__ -m64
+AFLAGS          += -m64
+CFLAGS          += -m64
 
 cflags-y	:=
 cflags-kernel-y	:=
@@ -36,7 +38,6 @@ cflags-$(CONFIG_MCORE2) += \
 	$(call cc-option,-march=core2,$(call cc-option,-mtune=generic))
 cflags-$(CONFIG_GENERIC_CPU) += $(call cc-option,-mtune=generic)
 
-cflags-y += -m64
 cflags-y += -mno-red-zone
 cflags-y += -mcmodel=kernel
 cflags-y += -pipe
@@ -69,7 +70,6 @@ cflags-$(CONFIG_CC_STACKPROTECTOR_ALL) += $(shell $(CONFIG_SHELL) $(srctree)/scr
 
 CFLAGS += $(cflags-y)
 CFLAGS_KERNEL += $(cflags-kernel-y)
-AFLAGS += -m64
 
 head-y := arch/x86_64/kernel/head.o arch/x86_64/kernel/head64.o arch/x86_64/kernel/init_task.o
 
-
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