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

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Date:	Thu, 06 Jan 2011 16:10:55 +0000
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Frederic Weisbecker" <fweisbec@...il.com>
Cc:	"H. Peter Anvin" <a.p.zijlstra@...llo.nl>,
	"Ingo Molnar" <mingo@...e.hu>,
	"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

>>> On 06.01.11 at 16:45, Frederic Weisbecker <fweisbec@...il.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

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