linux-kernel - Re: [PATCH v2 2/2] x86,vdso: Fix cross-compilation from big-endian architectures

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Date:	Fri, 30 May 2014 13:02:55 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Paul Gortmaker <paul.gortmaker@...driver.com>
CC:	Josh Boyer <jwboyer@...oraproject.org>,
	Ingo Molnar <mingo@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] x86,vdso: Fix cross-compilation from big-endian
 architectures

On 05/30/2014 08:48 AM, Andy Lutomirski wrote:
> This adds a macro GET(x) to convert x from big-endian to
> little-endian.  Hopefully I put it everywhere it needs to go and got
> all the cases needed for everyone's linux/elf.h.
> 
> Signed-off-by: Andy Lutomirski <luto@...capital.net>
> ---
>  arch/x86/vdso/vdso2c.c | 15 ++++++++++++
>  arch/x86/vdso/vdso2c.h | 63 ++++++++++++++++++++++++++++----------------------
>  2 files changed, 50 insertions(+), 28 deletions(-)

A couple of observations:

1. We shouldn't use double-underscore in host C code.

2. It would be nice if we can take these sort of things (host-build
   helper macros) and move them to some common file in the Linux kernel
   eventually, so it would be a good thing to make the naming a little
   less general.

3. Even though it isn't necessary, making it work on 8-bit values so
   one doesn't have to worry about the type would seem like a good
   thing.

I came up with the following, it seems like a reasonable simplification:

> #define _LE(x, bits, ifnot)						\
> 	__builtin_choose_expr(						\
> 		(sizeof(x) == bits/8),					\
> 		(__typeof__(x))le##bits##toh(x), ifnot)
> 
> extern void bad_le(uint64_t);
> #define _LAST_LE(x)							\
> 	__builtin_choose_expr(sizeof(x) == 1, (x), bad_le(x))
> 
> #define LE(x)								\
> 	_LE(x, 64, _LE(x, 32, _LE(x, 16, _LAST_LE(x))))

What do you think?

	-hpa

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