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Date: Thu, 06 Jan 2011 16:10:55 +0000
From: "Jan Beulich" <JBeulich@novell.com>
To: "Frederic Weisbecker" <fweisbec@gmail.com>
Cc: "H. Peter Anvin" <a.p.zijlstra@chello.nl>, "Ingo Molnar" <mingo@elte.hu>,
        "Stephane Eranian" <eranian@google.com>,
        "Thomas Gleixner" <tglx@linutronix.de>,
        "Arnaldo Carvalho de Melo" <acme@redhat.com>,
        "Soeren Sandmann Pedersen" <sandmann@redhat.com>,
        "LKML" <linux-kernel@vger.kernel.org>
Subject: Re: [RFC PATCH 1/2] x86: Fix rbp saving in pt_regs on irq
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>>> On 06.01.11 at 16:45, Frederic Weisbecker <fweisbec@gmail.com> wrote:
> Before we had:
> 
> 
> 	leaveq
> 
> 	CFI_RESTORE             rbp
> 	CFI_DEF_CFA_REGISTER    rsp
> 	CFI_ADJUST_CFA_OFFSET   -8
> 
> So CFI_RESTORE means rbp has now the value of the base frame of
> the calling frame (the base frame pointer of the interrupted proc) ?

No - all it means is that %rbp now has its original (caller or
interrupted procedure) value again (i.e. an unwinder should not
try to read it from the stack [or other previously recorded
location] anymore).

> And what follows means that rsp-8 points to the return address?

No - .cfi_def_cfa_register says which register serves as the frame
pointer, and .cfi_adjust_cfa_offset says to adjust the offset from
the frame pointer to the top [or bottom] of frame. At any time

	CFA = cfa_register + cfa_offset

with CFA being what all locations on the stack are expressed
relative to.

Jan