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 13:34:37 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Paul Gortmaker <paul.gortmaker@...driver.com>,
	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 Fri, May 30, 2014 at 1:21 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 05/30/2014 01:09 PM, Andy Lutomirski wrote:
>>>
>>> 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)
>>
>> This will do awful things if x is a floating-point type, and, for
>> integers, the cast is probably unnecessary.  But it should be okay.
>>
>
> I mostly wanted to preserve the signedness.  Yes, if we care about
> floating-point it gets trickier.
>
> At some point hopefully there will be a native C feature to handle this
> crap.
>
>>>> extern void bad_le(uint64_t);
>>
>> If this ever goes in a common header, then we should do the
>> __attribute__((error)) thing.  I wonder if it would ever make sense to
>> have __LINUX_HOSTPROG__ and make some of the basic headers work.  Hmm.
>>
>>>> #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?
>>
>> My only real real objection is that _LE sounds like converting *to*
>> little-endian to me.  Admittedly, that's the same thing on any
>> remotely sane architecture, but still.
>
> GET_LE() then?

Sounds good.

Are you planning on writing the patch?

I think my v2 is good -- the only diff I could find in my image.c
files and Stephen's was in the alt_xyz output, and I think I fixed
that in v2.

--Andy
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