[PATCH] Re: Hardcoded instruction causes certain features to fail on ARM platfrom due to endianness

Discussion:

Fei Yang

2012-10-15 15:33:08 UTC

Hi all,
I found that hardcoded instruction in inline asm can cause certains certain features fail to work on ARM platform due to endianness.
/ *
* here's the WFI
*/
asm(".word 0xe320f003\n"
: "memory", "cc");
The instruction generated from this inline asm will not work on big-endian ARM platform, such as ARM BE-8 format. Instead, an exception will be generated.
/ *
* here's the WFI
*/
asm("WFI\n"
: "memory", "cc");
Seems the kernel doesn't support ARM BE-8 well. I don't know why this problem happens.
Can anyone tell me who owns this part? I can prepare a patch then.
Thanks.

Questions regarding the ARM kernel should go to the linux-arm-kernel mailing list
(see the MAINTAINERS file), with an optional cc: to the regular LKML.
BE-8 is, if I recall correctly, ARMv7's broken format where code and data have
different endianess. GAS supports an ".inst" directive which is like ".word"
except the data is assumed to be code. This matters for disassembly, and may
also be required for BE-8.
That is, just s/.word/.inst/g above and report back if that works or not.
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Hi Mikael,

Thanks for the reply. I modified the code as suggested and rebuilt the
kernel, cpu-hotplug feature now works on big-endian(BE-8) ARM
platform.
Since the ARM core can be configured by system software to work in
big-endian mode, it's necessary to fix this problem. And here is a
small patch :

diff -urN linux-3.6.2/arch/arm/mach-exynos/hotplug.c
linux/arch/arm/mach-exynos/hotplug.c
--- linux-3.6.2/arch/arm/mach-exynos/hotplug.c 2012-10-13
04:50:59.000000000 +0800
+++ linux/arch/arm/mach-exynos/hotplug.c 2012-10-15 23:05:44.000000000 +0800
@@ -72,7 +72,7 @@
/*
* here's the WFI
*/
- asm(".word 0xe320f003\n"
+ asm(".inst 0xe320f003\n"
:
:
: "memory", "cc");
diff -urN linux-3.6.2/arch/arm/mach-realview/hotplug.c
linux/arch/arm/mach-realview/hotplug.c
--- linux-3.6.2/arch/arm/mach-realview/hotplug.c 2012-10-13
04:50:59.000000000 +0800
+++ linux/arch/arm/mach-realview/hotplug.c 2012-10-15 23:05:00.000000000 +0800
@@ -66,7 +66,7 @@
/*
* here's the WFI
*/
- asm(".word 0xe320f003\n"
+ asm(".inst 0xe320f003\n"
:
:
: "memory", "cc");
diff -urN linux-3.6.2/arch/arm/mach-shmobile/hotplug.c
linux/arch/arm/mach-shmobile/hotplug.c
--- linux-3.6.2/arch/arm/mach-shmobile/hotplug.c 2012-10-13
04:50:59.000000000 +0800
+++ linux/arch/arm/mach-shmobile/hotplug.c 2012-10-15 23:05:25.000000000 +0800
@@ -53,7 +53,7 @@
/*
* here's the WFI
*/
- asm(".word 0xe320f003\n"
+ asm(".inst 0xe320f003\n"
:
:
: "memory", "cc");
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Dave Martin

2012-10-16 12:49:56 UTC

Permalink

Post by Fei Yang

The cleanest fix would simply be to build these files with appropriate
modified CFLAGS (-march=armv6k or -march=armv7-a), and use the proper
"wfi" mnemonic.

Failing that, you could use the facilities in <asm/opcodes.h> to
declare a wrapper macro for injecting this opcode (see
<asm/opcodes-virt.h> for an example). However, putting custom
opcodes into the assembler should only be done if it's really
necessary. Nowadays, I think we can consider tools which don't
understand the WFI mnemonic to be obsolete, at least for platforms
which only build for v7 and above.

The relevant board maintainers would need to sign off on such a
change, so we don't end up breaking their builds.

If any of these boards needs to build for v6K, the custom opcode might
be worth it -- some people might just possibly be relying on older tools
for such platforms.

Cheers
---Dave

Post by Fei Yang
: "memory", "cc");
diff -urN linux-3.6.2/arch/arm/mach-realview/hotplug.c
linux/arch/arm/mach-realview/hotplug.c
--- linux-3.6.2/arch/arm/mach-realview/hotplug.c 2012-10-13
04:50:59.000000000 +0800
+++ linux/arch/arm/mach-realview/hotplug.c 2012-10-15 23:05:00.000000000 +0800
@@ -66,7 +66,7 @@
/*
* here's the WFI
*/
- asm(".word 0xe320f003\n"
+ asm(".inst 0xe320f003\n"
: "memory", "cc");
diff -urN linux-3.6.2/arch/arm/mach-shmobile/hotplug.c
linux/arch/arm/mach-shmobile/hotplug.c
--- linux-3.6.2/arch/arm/mach-shmobile/hotplug.c 2012-10-13
04:50:59.000000000 +0800
+++ linux/arch/arm/mach-shmobile/hotplug.c 2012-10-15 23:05:25.000000000 +0800
@@ -53,7 +53,7 @@
/*
* here's the WFI
*/
- asm(".word 0xe320f003\n"
+ asm(".inst 0xe320f003\n"
: "memory", "cc");
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Fei Yang

2012-10-16 16:33:54 UTC

Permalink

Post by Dave Martin

Post by Fei Yang

The cleanest fix would simply be to build these files with appropriate
modified CFLAGS (-march=armv6k or -march=armv7-a), and use the proper
"wfi" mnemonic.
Failing that, you could use the facilities in <asm/opcodes.h> to
declare a wrapper macro for injecting this opcode (see
<asm/opcodes-virt.h> for an example). However, putting custom
opcodes into the assembler should only be done if it's really
necessary. Nowadays, I think we can consider tools which don't
understand the WFI mnemonic to be obsolete, at least for platforms
which only build for v7 and above.
The relevant board maintainers would need to sign off on such a
change, so we don't end up breaking their builds.
If any of these boards needs to build for v6K, the custom opcode might
be worth it -- some people might just possibly be relying on older tools
for such platforms.
Cheers
---Dave

Thanks for the suggestions. The ".inst" directive here may also bring
us trouble if some older tools is used.
In that situation, "wfi" mnemonic will not be recognized either. If we
cannot suppose that newer tools is used, then how can we determine the
endianness during the preprocessor/compile phase? Any ideas?

BTW: I found this bug on my ARM V7-A Cortex-A9 board and the processor
is configured to work in big-endian mode at boot stage (word and
halfword data is interpreted as big-endian, but instruction is still
little-endian) . The kernel is ported from arch/arm/mach-realview. And
I think these boards(mach-realview/mach-shmobile/mach-exynos) should
have the similar problems. ARM arch is Bi-endian since versions 3 and
above.
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Dave Martin

2012-10-16 17:25:15 UTC

Permalink

Post by Fei Yang

Post by Dave Martin

Post by Fei Yang

The cleanest fix would simply be to build these files with appropriate
modified CFLAGS (-march=armv6k or -march=armv7-a), and use the proper
"wfi" mnemonic.
Failing that, you could use the facilities in <asm/opcodes.h> to
declare a wrapper macro for injecting this opcode (see
<asm/opcodes-virt.h> for an example). However, putting custom
opcodes into the assembler should only be done if it's really
necessary. Nowadays, I think we can consider tools which don't
understand the WFI mnemonic to be obsolete, at least for platforms
which only build for v7 and above.
The relevant board maintainers would need to sign off on such a
change, so we don't end up breaking their builds.
If any of these boards needs to build for v6K, the custom opcode might
be worth it -- some people might just possibly be relying on older tools
for such platforms.
Cheers
---Dave

The endianness is controlled by the build-time configuration of the
kernel. A single kernel image cannot be bi-endian.

The __inst_*() macros in <asm/opcodes.h> take care of this based on which
CONFIG_CPU_ENDIAN_* option is selected by the board in the kernel config.
For compatibility with old tools, this is done instead of using the
".inst" directive.

Post by Fei Yang
BTW: I found this bug on my ARM V7-A Cortex-A9 board and the processor
is configured to work in big-endian mode at boot stage (word and
halfword data is interpreted as big-endian, but instruction is still
little-endian) . The kernel is ported from arch/arm/mach-realview. And
I think these boards(mach-realview/mach-shmobile/mach-exynos) should
have the similar problems. ARM arch is Bi-endian since versions 3 and
above.

I believe that shmobile and exynos are v7-only, so it may be better to
just use "wfi" and override the CFLAGS for those files. As you can
see, those were just created by copy-pasting the code from mach-realview.

realview itself can be used with ARMv6 based core-tiles, so there may be
an argument for a custom opcode in this case:

#include <asm/opcodes.h>
#define __WFI __inst_arm_thumb16(0xE320F003, 0xBF30)

Not handling the Thumb case is a definite bug for any file which may
run on v7, since the kernel could be built in Thumb for that case.
For example, the existing code is mach-realview/hotplug.c is broken
when building an SMP Thumb-2 kernel for the Realview PBX-A9.

Cheers
---Dave
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Yangfei (Felix)

2012-10-15 10:28:22 UTC

Permalink

Hi Mikael & Russell,

Thanks for the suggestions. I tried to replace ".word" with ".inst" and the cpu-hotplug feature now works on big-endian (BE-8) ARM platform. And illegal instruction exception disappears.

And the following is my small patch to fix the problem,

diff -urN linux-3.7-rc1/arch/arm/mach-exynos/hotplug.c linux/arch/arm/mach-exynos/hotplug.c
--- linux-3.7-rc1/arch/arm/mach-exynos/hotplug.c 2012-10-15 05:41:04.000000000 +0800
+++ linux/arch/arm/mach-exynos/hotplug.c 2012-10-15 17:47:28.177103266 +0800
@@ -72,7 +72,7 @@
/*
* here's the WFI
*/
- asm(".word 0xe320f003\n"
+ asm(".inst 0xe320f003\n"
:
:
: "memory", "cc");
diff -urN linux-3.7-rc1/arch/arm/mach-realview/hotplug.c linux/arch/arm/mach-realview/hotplug.c
--- linux-3.7-rc1/arch/arm/mach-realview/hotplug.c 2012-10-15 05:41:04.000000000 +0800
+++ linux/arch/arm/mach-realview/hotplug.c 2012-10-15 17:44:55.915942976 +0800
@@ -64,7 +64,7 @@
/*
* here's the WFI
*/
- asm(".word 0xe320f003\n"
+ asm(".inst 0xe320f003\n"
:
:
: "memory", "cc");
diff -urN linux-3.7-rc1/arch/arm/mach-shmobile/hotplug.c linux/arch/arm/mach-shmobile/hotplug.c
--- linux-3.7-rc1/arch/arm/mach-shmobile/hotplug.c 2012-10-15 05:41:04.000000000 +0800
+++ linux/arch/arm/mach-shmobile/hotplug.c 2012-10-15 17:45:20.509879863 +0800
@@ -39,7 +39,7 @@
/*
* here's the WFI
*/
- asm(".word 0xe320f003\n"
+ asm(".inst 0xe320f003\n"
:
:
: "memory", "cc");

Since ARM processor can be configured to work in BE-8 big-endian, I thinks this patch is necessary. Any suggestions is welcomed.
Thanks

Questions regarding the ARM kernel should go to the linux-arm-kernel mailing list
(see the MAINTAINERS file), with an optional cc: to the regular LKML.

BE-8 is, if I recall correctly, ARMv7's broken format where code and data have
different endianess. GAS supports an ".inst" directive which is like ".word"
except the data is assumed to be code. This matters for disassembly, and may
also be required for BE-8.

That is, just s/.word/.inst/g above and report back if that works or not.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to ***@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/