linux-kernel - Re: Duplication of vdso and vsyscall code?

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Date:	Fri, 09 Oct 2009 21:47:28 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Andi Kleen <andi@...stfloor.org>
CC:	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>
Subject: Re: Duplication of vdso and vsyscall code?

On 10/09/09 21:14, Andi Kleen wrote:
> One is at a fixed position in the user address space, and the other at a
> randomized position. The fixed one came first. Fixed doesn't know
> where randomized is. Randomized is also compiled and linked completely
> differently.
Is the fixed vsyscall stuff now considered to be legacy?  As far as I
can see, the vdso seems to be what ends up being used on all the systems
I've tried (going back to Fedora 8).

>  In theory the randomized one could call the fixed one, but
> originally there were some thoughts about turning off fixed for some
> applications that don't need it and also the path was considered very
> performance critical, so unneeded jumps were avoided.
>   

rdtsc seems to swamp pretty much everything else.  In my measurements it
alone takes 1/3 of the time.  Though that's Core2; AMD have
traditionally been much better at those kinds of things.

> In theory you could probably #include the code from a common file, but it 
> wouldn't buy you too much.

Yes, that's what I had in mind.  I don't think duplicating the
instructions is all that important, but having two separate similarish
pieces of code seems like a maintenance headache.  I only discovered the
second set of code by accident; I'd assumed that once I'd found one
vgettimeofday I'd found them all (and I'd been assuming that
clock_gettime didn't get the same treatment).

I'll see what happens if I try unifying them...

    J
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