linux-kernel - Re: [PATCH] arm: Add unwinding annotations for 64bit division functions

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Date:	Wed, 21 Sep 2011 12:55:53 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Dave Martin <dave.martin@...aro.org>
Cc:	Laura Abbott <lauraa@...eaurora.org>,
	Nicolas Pitre <nico@...xnic.net>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org, linux-arm-msm@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm: Add unwinding annotations for 64bit division
	functions

On Wed, Sep 21, 2011 at 12:39:09PM +0100, Dave Martin wrote:
> Talking to Catalin a bit more, it sounds like prefetch aborts should not
> happen in kernel code, and data aborts should not happen when accessing
> the kernel stack.

No faults should happen in kernel code, except for:

1. instructions specifically marked in the exception table, which are used
   to access user memory.
2. instructions causing an 'undefined instruction' exception.

Standard ARM instructions like 'add', 'mov' etc should _never_ fault,
and if they do that means your core isn't executing ARM instructions
correctly (eg, the hardware design is faulty.)

Instructions such as VFP, kprobes tracing, etc are expected fault
locations, and those are fairly well controlled where they can be placed.
With things like ftrace, it certainly is the case that the unwinder can
theoretically be called from almost anywhere in a function.

So I suggest that this does need to be fixed, and you can't rely on
"prefetch aborts should not happen".  That's true of prefetch aborts
but not of other aborts.
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