From: Laurent Vivier <laurent@vivier.eu>
To: Scott Wood <scottwood@freescale.com>
Cc: benh@kernel.crashing.org, paulus@samba.org,
linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] powerpc: allow cross-compilation of ppc64 kernel
Date: Fri, 6 Nov 2015 23:22:11 +0100 [thread overview]
Message-ID: <563D2813.80302@vivier.eu> (raw)
In-Reply-To: <1446844180.11597.13.camel@freescale.com>
Le 06/11/2015 22:09, Scott Wood a écrit :
> On Thu, 2015-11-05 at 12:47 +0100, Laurent Vivier wrote:
>> When I try to cross compile a ppc64 kernel, it generally
>> fails on the VDSO stage. This is true for powerpc64 cross-
>> compiler, but also when I try to build a ppc64le kernel
>> on a ppc64 host.
>>
>> VDSO64L fails:
>>
>> VDSO64L arch/powerpc/kernel/vdso64/vdso64.so.dbg
>> /usr/bin/powerpc64-linux-gnu-ld: arch/powerpc/kernel/vdso64/sigtramp.o:
>> file class ELFCLASS64 incompatible with ELFCLASS32
>> /usr/bin/powerpc64-linux-gnu-ld: final link failed: File in wrong format
>>
>> This fails because gcc calls "collect2" with
>> "--oformat elf32-powerpcle" with ppc64 objects, without the
>> "--oformat" ld works well because it use the format of the
>> first object as output format.
>>
>> As this case is correctly managed to build the other kernel
>> objects, this patch replaces $(GCC) by $(LD) to generate the
>> VDSO objects.
>
> I cross-compile ppc64 kernels and have not seen this problem. I do need to
> pass in -m64 as part of $(CC) if it's not the toolchain default, which is not
> nice, but the proper fix for that is to add -m64 in the makefiles -- and if I
> don't it fails way before VDSO.
>
> Why is GCC building ppc64 object files but telling the linker --oformat elf32-
> powerpcle? Are different options somehow being passed to GCC in one case
> versus the other?
In fact, for all the other parts of the kernel, gcc is called with
"-mlittle-endian -m64", ld with "-EL -m elf64lppc", and thus generates
the good objects and calls ld with the good options ("elf64lppc"). I
think gcc is never used to link, only to compile.
This, I think, comes from:
arch/powerpc/Makefile:
ifeq ($(CONFIG_CPU_LITTLE_ENDIAN),y)
override CC += -mlittle-endian
override LD += -EL
...
ifeq ($(HAS_BIARCH),y)
override CC += -m$(CONFIG_WORD_SIZE)
override LD += -m elf$(CONFIG_WORD_SIZE)$(LDEMULATION)
But in the case of vdso64, ld command is gcc:
arch/powerpc/kernel/vdso64/Makefile:
cmd_vdso64ld = $(CC) $(c_flags) -Wl,-T $^ -o $@
So to link, we use "gcc -mlittle-endian -m64"
and strace of "gcc -mlittle-endian -m64" gives me:
"/usr/libexec/gcc/ppc64-redhat-linux/5.1.1/collect2", [ ... "--oformat",
"elf32-powerpcle", "-m", "elf64lppc",...
So the format used to link is by default "elf32-powerpcle" (with the
emulation elf64lppc given by "-mlittle-endian -m64", I agree it seems
strange).
I think this is coming from the configuration of my gcc, "-dumpspecs"
gives me:
*link_target:
%{mlittle|mlittle-endian: --oformat
elf32-powerpcle;mbig|mbig-endian:;mcall-i960-old: --oformat
elf32-powerpcle;:}
When "ld" is called without "--oformat" it takes the format of the first
processed object ("elf64-powerpcle" in our case, it's why it works for
the other binaries of the kernel).
So at this point, I can:
1- either fix my compiler,
2- or fix the vdso64 linker command.
As all the others objects of the kernel are generated and linked
correctly with this build env, there is no reason to not choose 2- as 1-
is much more complex (at leasts for me = rebuild gcc whereas I just want
to build the kernel).
But, more generally, what I'm wondering is why we are using CC instead
of LD to link objects...
Laurent
next prev parent reply other threads:[~2015-11-06 22:22 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-05 11:47 [PATCH] powerpc: allow cross-compilation of ppc64 kernel Laurent Vivier
2015-11-06 21:09 ` Scott Wood
2015-11-06 22:22 ` Laurent Vivier [this message]
2015-11-06 23:24 ` Scott Wood
2015-11-07 11:35 ` Laurent Vivier
2015-11-10 0:29 ` Michael Ellerman
2015-11-10 7:54 ` Laurent Vivier
2015-11-06 22:55 ` Segher Boessenkool
2015-11-06 23:32 ` Segher Boessenkool
2015-11-07 11:32 ` Laurent Vivier
2015-11-06 23:12 ` Benjamin Herrenschmidt
2015-11-07 11:34 ` Laurent Vivier
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