linux-kernel - Re: [PATCH 0/6][RFC] Rework vsyscall to avoid truncation/rounding issue in timekeeping core

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Date:	Wed, 19 Sep 2012 09:31:35 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Richard Cochran <richardcochran@...il.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Tony Luck <tony.luck@...el.com>,
	Paul Mackerras <paulus@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Paul Turner <pjt@...gle.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Prarit Bhargava <prarit@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/6][RFC] Rework vsyscall to avoid truncation/rounding
 issue in timekeeping core

On 09/18/2012 09:50 PM, Richard Cochran wrote:
> On Tue, Sep 18, 2012 at 11:29:50AM -0700, John Stultz wrote:
>> I believe its mostly historical, but on some architectures that
>> history has become an established ABI, making it technical.
> Fine, but what do you mean by "ABI?" Are you talking about magic
> addresses for functions?
On powerpc, I mean magic addresses where userland can find structures 
that it can use to calculate time.

On ia64 I mean the fsyscall method (which is arch specific).

> Without knowing the dirty details, what I imagine is a jump/branch
> from the arch-specific code into the common implementation.
>
> Can that be done?
In the two cases above, what you suggest unfortunately isn't possible 
(at least to my understanding - arch maintainers jump in to correct me).

With powerpc, there is no arch specific kernel code involved, its just a 
data structure the kernel exports that is accessible to userland. The 
execution logic lives in userland libraries, or sometimes application 
code itself.

With ia64's fsyscall, its a special mode that limits what you can do and 
which registers you access. So you couldn't just jump to other code 
while in that mode.

But maybe someone has a neat idea on how to get around this?

thanks
-john

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