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

[Robin Murphy]

Hi Romain,

On 20/09/17 16:18, Romain Izard wrote:
> 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.

Nit: that sentence is not really true - there are CPUs without MMUs that
still support alignment checking (e.g. ARM1156), and more importantly
the rotation thing is only true for LDR/LDRH - LDM always just ignores
the bottom two bits of the address, and LDRD doesn't make any guarantee
at all of what you might get back (and stores are even worse). I think
it suffices to say that in the absence of alignment checking, the
results of unaligned accesses are inconsistent and impossible to rely upon.

Robin.

> 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@xxxxxxxxx>
> ---
> 
> 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 */
>