linux-kernel - Re: [RFC PATCH 1/2] x86: Fix rbp saving in pt

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Date:	Fri, 7 Jan 2011 13:31:30 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	"H. Peter Anvin" <a.p.zijlstra@...llo.nl>,
	Stephane Eranian <eranian@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Soeren Sandmann Pedersen <sandmann@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 1/2] x86: Fix rbp saving in pt_regs on irq entry

* Jan Beulich <JBeulich@...ell.com> wrote:

> > Now I don't understand how this is all useful as this is not a normal proc but 
> > an interruption. We can't get back the return address from the CFA. Or am I 
> > missing something?
> 
> Unwind annotations, when written correctly, allow unwinding through all kinds of 
> execution flows, including interrupts or exceptions as well as including stack 
> switches.

Yeah and that's rather useful, as exception contexts can nest in very weird ways, 
especially with NMIs involved. For example a 7-context combination is possible:

  user-space -> syscall -> pagefault -> softirq -> hardirq -> debug trap -> NMI

And the call frame walking logic needs to be able to get all the way back to 
user-space ...

For that every transition needs to work flawlessly, for debugging (and CFI based 
profiling) to work fine.

Most of those transitions can happen at any instruction boundary that a given 
context executes, so the total number of possible combinations is virtually endless.

Unfortunately we dont seem to have a good way to test any of this automatically. 
Putting a perf probe on every assembly instruction perhaps, and checking whether the 
frame manages to go back all the way to user-space?

Thanks,

	Ingo
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