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Date: Mon, 11 Aug 2014 15:26:14 +0200
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Jan Beulich <JBeulich@suse.com>, Frederic Weisbecker <fweisbec@gmail.com>,
        Denys Vlasenko <vda.linux@googlemail.com>
CC: Andy Lutomirski <luto@amacapital.net>, Kees Cook <keescook@chromium.org>,
        Will Drewry <wad@chromium.org>, X86 ML <x86@kernel.org>,
        Alexei Starovoitov <ast@plumgrid.com>, Oleg Nesterov <oleg@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH 4/5] x86: entry_64.S: always allocate complete "struct
 pt_regs"
References: <1406904498-21647-1-git-send-email-dvlasenk@redhat.com> <1406904498-21647-4-git-send-email-dvlasenk@redhat.com> <20140801231929.GB26491@localhost.localdomain> <CAK1hOcNXdBwd4KZi1V2dLMtRdusvNLPZwTKwYvnpvz3PRnsM+Q@mail.gmail.com> <20140811004559.GA2656@lerouge> <53E89DA3020000780002B00B@mail.emea.novell.com>
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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?