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

H. Peter Anvin
2014-05-30T20:03:32Z

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@amacapital.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