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

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Message-ID: <20110106172355.GE2308@nowhere>
Date:	Thu, 6 Jan 2011 18:24:02 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jan Beulich <JBeulich@...ell.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 Thu, Jan 06, 2011 at 06:12:33PM +0100, Frederic Weisbecker wrote:
> On Thu, Jan 06, 2011 at 04:58:54PM +0000, Jan Beulich wrote:
> > >>> On 06.01.11 at 17:54, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > > On Thu, Jan 06, 2011 at 04:39:39PM +0000, Jan Beulich wrote:
> > >> >>> On 06.01.11 at 17:22, Frederic Weisbecker <fweisbec@...il.com> wrote:
> > >> > On Thu, Jan 06, 2011 at 04:10:55PM +0000, Jan Beulich wrote:
> > >> >> >>> 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.
> > >> > 
> > >> > Ok.
> > >> > 
> > >> > So here rsp points to pt_regs::r11
> > >> > 
> > >> > I don't understand why locations relative to the stack must be
> > >> > expressed here by taking rsp - 8 as a base.
> > >> 
> > >> Nothing says rsp-8. The annotations merely say to set the base
> > >> register to rsp and to *adjust* the offset by -8 (after all, that's
> > >> what the leaveq instruction does).
> > > 
> > > Ah! So CFA acts like a virtual frame base pointer right?
> > 
> > Correct.
> 
> Ah great. I was starting to prepare for the case you come to stab me :)
> 
> So what do you think about that:
> 
>         leaveq
> 
>         CFI_RESTORE             rbp
>         CFI_DEF_CFA_REGISTER    rsp
>         CFI_ADJUST_CFA_OFFSET   -8
> 
>        /* we did not save rbx, restore only from ARGOFFSET */
>        addq $8, %rsp
>        CFI_ADJUST_CFA_OFFSET   -16

Or if CFI_ADJUST_CFA_OFFSET is already relative to its previous value,
it should be CFI_ADJUST_CFA_OFFSET -8
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