linux-kernel - Re: [PATCH] Turn rdmsr, rdtsc into inline functions, clarify names

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Date:	Sun, 06 Aug 2006 12:56:03 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Andi Kleen <ak@....de>
Cc:	lkml - Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Turn rdmsr, rdtsc into inline functions, clarify names

On Sun, 2006-08-06 at 04:38 +0200, Andi Kleen wrote:
> On Sat, Aug 05, 2006 at 07:47:41PM +1000, Rusty Russell wrote:
> > [Andi, sorry, x86_64 part untested, so sending straight to you]
> > 
> > rdmsr and rdtsc are macros, altering their arguments directly.  An
> > inline function would offer decent typechecking, but needs to take
> > pointer args.  The comment notes that gcc produces better code with
> 
> I think I prefer the macro variant actually. Sorry. It just looks
> better without the &s.

Hi Andi,

	Please reconsider.  This isn't about being pretty, it's about not
having hidden side-effects, and having typechecking.

> We don't care very much about the code quality here because
> rdmsr/wrmsr are always very slow in microcode anyways and tend
> to synchronize the CPUs.

Agreed, but comment about it above the macros made me wary, so I checked
it.  No significant code difference with gcc >= 4.0, at least.

> If you feel a need to clean up I would suggest you convert more
> users over to the ll variants which take a single 64bit value
> instead of two 32bit ones.

You mean the l and ll variants?  The 64 bit variants are rdmsrl and
rdtscll, not to be confused with rdtscl, which returns the lower 32
bits.  This confusion caused the x86_64 bug in gameport.c which the
patch comment mentioned (at least, seems to be a bug to me).

See why I want to fix these names?

So if you would prefer u64 rdtsc64(), u32 rdtsc_low(), u64 rdmsr64(int
msr), u32 rdmsr_low(int msr), I can convert everyone to that, although
it's a more invasive change...

Thanks,
Rusty.
-- 
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