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

Jan Beulich
2011-01-06T16:39:44Z

>>> On 06.01.11 at 17:22, Frederic Weisbecker <fweisbec@gmail.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@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.

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).

Jan