Re: [PATCH 4/5] x86: entry_64.S: always allocate complete "struct pt_regs"

["Jan Beulich" <JBeulich@suse.com>]

>>> On 11.08.14 at 15:26, <dvlasenk@redhat.com> wrote:
> On 08/11/2014 10:40 AM, Jan Beulich wrote:
>>>>>>       CFI_ESCAPE      0x0f /* DW_CFA_def_cfa_expression */, 6, \
>>>>>>                       0x77 /* DW_OP_breg7 */, 0, \
>>>>>>                       0x06 /* DW_OP_deref */, \
>>>>>> -                     0x08 /* DW_OP_const1u */, SS+8-RBP, \
>>>>>> +                     0x08 /* DW_OP_const1u */, SS+8, \
>>>>>>                       0x22 /* DW_OP_plus */
>>>>>>       /* We entered an interrupt context - irqs are off: */
>>>>>>       TRACE_IRQS_OFF
> ...
> ...
>>>> While we are at it, what this  CFI_ESCAPE thing does here?
>>>> As usual, it has no comment :/
>> 
>> Each of its lines has a comment; with other CFI annotations not
>> each having comments, I don't see what else is needed here.
> 
> The existing comments explain what every byte means.
> They are useful if CFI-literate reader wants to check correctness
> of the encoding of this annotation.
> 
> There is no overall comment what this CFI annotation
> *achieves*. In human language, what do we say
> to DWARF decoder here?

Short answer: DW_CFA_def_cfa_expression.

Longer response: Just like I said before, what you're asking for is
identical to ask for each other CFI annotation to get a comment
associated to tell you what it's doing, which I don't think you
really mean to ask for. (Our main problem here is that we can't
specify expressions with the .cfi_* gas directives, and hence have
to resort to .cfi_escape.)

Jan