linux-kernel - [PATCH] ARM: unaligned.h: Use an arch-specific version

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Message-Id: <20170920151802.7609-1-romain.izard.pro@gmail.com>
Date:   Wed, 20 Sep 2017 17:18:02 +0200
From:   Romain Izard <romain.izard.pro@...il.com>
To:     Russell King <linux@...linux.org.uk>,
        Al Viro <viro@...iv.linux.org.uk>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Romain Izard <romain.izard.pro@...il.com>
Subject: [PATCH] ARM: unaligned.h: Use an arch-specific version

For the 32-bit ARM architecture, unaligned access support is variable.
On a chip without a MMU, an unaligned access returns a rotated data word
and must be avoided.

When a MMU is available, it can be trapped. On ARMv6 or ARMv7, it can also
be handled by the hardware, depending on the type of access instruction.
Unaligned access of 32 bits or less are corrected, while larger access
provoke a trap.

Unfortunately, the compiler is able to merge two 32-bit access that
would generate a LDR instruction, that works on unaligned access, into a
single LDM access that will not work. This is not a common situation,
but it has been observed in the LZ4 decompression code.

To prevent this type of optimization, it is necessary to change the
explicit accessors for unaligned addresses from those defined in the
access_ok.h header, to those defined in the packed_struct.h header.

Add an arch-specific header to ARM, to retain other optimizations that
rely on HAVE_EFFICIENT_UNALIGNED_ACCESS, while making sure that access
that explicitly rely on the unaligned accessors are correctly handled by
the compiler.

Signed-off-by: Romain Izard <romain.izard.pro@...il.com>
---

This is a follow-up to this discussion:
HAVE_EFFICIENT_UNALIGNED_ACCESS on ARM32
https://lkml.org/lkml/2017/9/4/359

 arch/arm/include/asm/Kbuild      |  1 -
 arch/arm/include/asm/unaligned.h | 22 ++++++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)
 create mode 100644 arch/arm/include/asm/unaligned.h

diff --git a/arch/arm/include/asm/Kbuild b/arch/arm/include/asm/Kbuild
index 721ab5ecfb9b..0f2c8a2a8131 100644
--- a/arch/arm/include/asm/Kbuild
+++ b/arch/arm/include/asm/Kbuild
@@ -20,7 +20,6 @@ generic-y += simd.h
 generic-y += sizes.h
 generic-y += timex.h
 generic-y += trace_clock.h
-generic-y += unaligned.h

 generated-y += mach-types.h
 generated-y += unistd-nr.h
diff --git a/arch/arm/include/asm/unaligned.h b/arch/arm/include/asm/unaligned.h
new file mode 100644
index 000000000000..394227f24b77
--- /dev/null
+++ b/arch/arm/include/asm/unaligned.h
@@ -0,0 +1,22 @@
+#ifndef __ASM_ARM_UNALIGNED_H
+#define __ASM_ARM_UNALIGNED_H
+
+#include <asm/byteorder.h>
+
+#if defined(__LITTLE_ENDIAN)
+#include <linux/unaligned/le_struct.h>
+#include <linux/unaligned/be_byteshift.h>
+#include <linux/unaligned/generic.h>
+#define get_unaligned	__get_unaligned_le
+#define put_unaligned	__put_unaligned_le
+#elif defined(__BIG_ENDIAN)
+#include <linux/unaligned/be_struct.h>
:q
+#include <linux/unaligned/le_byteshift.h>
+#include <linux/unaligned/generic.h>
+#define get_unaligned	__get_unaligned_be
+#define put_unaligned	__put_unaligned_be
+#else
+#error need to define endianness
+#endif
+
+#endif /* __ASM_ARM_UNALIGNED_H */
-- 
2.11.0