linux-kernel - Re: [PATCH 6/6] perf_counter: tools: Makefile tweaks for 64-bit powerpc

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Message-ID: <19000.57673.618229.523229@cargo.ozlabs.ibm.com>
Date:	Wed, 17 Jun 2009 22:27:53 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	benh@...nel.crashing.org, Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org, linuxppc-dev@...abs.org
Subject: Re: [PATCH 6/6] perf_counter: tools: Makefile tweaks for 64-bit
	powerpc

Ingo Molnar writes:

> ah, it does this:
> 
> /*
>  * This is here because we used to use l64 for 64bit powerpc
>  * and we don't want to impact user mode with our change to ll64
>  * in the kernel.
>  */
> #if defined(__powerpc64__) && !defined(__KERNEL__)
> # include <asm-generic/int-l64.h>
> #else
> # include <asm-generic/int-ll64.h>
> #endif
> 
> That's crappy really.

We were concerned that changing the userland-visible type of __u64
from unsigned long to unsigned long long, etc., would be breaking the
ABI, even if only in a small way - I thought it could possibly change
C++ mangled function names, for instance, and it would cause fresh
compile warnings on existing user code that prints __u64 with %lx,
which has always been the correct thing to do on ppc64.

A counter-argument would be, I guess, that __u64 et al. are purely for
use in describing the kernel/user interface, so we have a little more
latitude than with the type of e.g. u_int64_t.  I dunno.  I don't
recall getting much of an answer from the glibc guys about what they
thought of the idea of changing it.

Anyway, of the 64-bit architectures, alpha, ia64, and mips64 also have
__u64 as unsigned long in userspace, so this issue will still crop up
even if we change it on powerpc.

Paul.
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