linux-kernel - Re: [PATCH] [1/3] Provide rdtscll() asm/msr.h for user space

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Date:	Tue, 07 Oct 2008 14:32:05 -0700
From:	"H. Peter Anvin" <hpa@...nel.org>
To:	Andi Kleen <andi@...stfloor.org>
CC:	x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [1/3] Provide rdtscll() asm/msr.h for user space

Andi Kleen wrote:
> For simple user space timing it's very useful to be able to do
> #include <asm/msr.h> instead of having to cut'n'paste the necessarily
> macros into all programs. This used to work with older kernels,
> but broke with the paravirt support (I ended up with a bunch of
> test programs that broke because of that). But it's very useful
> to have rdtscll() available somewhere in user space and asm/msr.h
> is a good place to have them.
> 
> Provide simple rdtsc/rdtscl() macros for user space too in asm/msr.h. 
> Since they are very simple I don't think they are a burden to maintain.
> 
> The diff looks bigger than it is because I moved a code block
> and diff doesn't handle it very well.

I really don't think this belongs in the kernel.  It's not even a case 
of "usable by accident" anymore, and hasn't worked for a while, so it's 
not a matter of legacy, either.

Mixing fundamentally unrelated kernel and userspace variants of the same 
function just makes the aggregation uglier than both.

(Also, most userspace variants I have seen have what the kernel calls 
"rdtscll" and calls it "rdtsc".)

I would suggest writing a <sys/tsc.h> header file and submitting to the 
glibc people, instead, or perhaps even better, start a libarch/libx86 tree.

	-hpa
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