linux-kernel - Re: [tip:x86/mce] x86/bitops: Move BIT

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Date:	Wed, 23 May 2012 09:50:47 -0700
From:	"H. Peter Anvin" <hpa@...nel.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Borislav Petkov <borislav.petkov@....com>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, frank.arnold@....com,
	akpm@...ux-foundation.org, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/mce] x86/bitops: Move BIT_64() for a wider use

On 05/23/2012 09:47 AM, H. Peter Anvin wrote:
> 
> BIT(0), okay.  I thought we were talking about BIT_64() here...
> 
> Any reason we can't just tell people to use BIT() for a native "unsigned
> long" type (32/64 bits) and BIT_64() if they really want a 64-bit result?
> 
> There are good reasons for the latter.  Consider, for example:
> 
> 	u64 msr;
> 	...
> 	msr &= ~BIT_64(1);
> 
> This *better* not be an unsigned 32 bit value, or we just chopped off
> the upper half.  In this case and similar ones the 64-bitness of the
> result really matters.
> 

To better clarify my concern: my concern is that if we make BIT() be a
DWIM type, it will appear to work in most situations.  As such, we'll
see things in headers like:

#define MSR_BLAH_FOO	BIT(31)
#define MSR_BLAH_BAR	BIT(32)

... and *almost all the time* the above will work.  But if you use
MSR_BLAH_FOO inverted, then you get truncation.

	-hpa

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