Re: [RFC PATCH 1/2] x86: Fix rbp saving in pt_regs on irq entry

Frederic Weisbecker
2011-01-06T16:22:44Z

On Thu, Jan 06, 2011 at 04:10:55PM +0000, Jan Beulich wrote:
> >>> 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.

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.