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From: Denys Vlasenko <vda.linux@googlemail.com>
Date: Sat, 10 Jan 2015 23:35:58 +0100
Message-ID: <CAK1hOcNgnZcF5fZjSSMr+RQQWLGBSjitErd4gj7JxgTMZPGTzQ@mail.gmail.com>
Subject: Re: [PATCH 3/4] x86: open-code register save/restore in
 trace_hardirqs thunks
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Denys Vlasenko <dvlasenk@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
        Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@amacapital.net>,
        Frederic Weisbecker <fweisbec@gmail.com>, X86 ML <x86@kernel.org>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>
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On Sat, Jan 10, 2015 at 11:07 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Sat, Jan 10, 2015 at 2:00 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>> +       pushq_cfi %rdi
>> +       CFI_REL_OFFSET rdi, 0
>> +       pushq_cfi %rsi
>> +       CFI_REL_OFFSET rsi, 0
>> +       pushq_cfi %rdx
>> +       CFI_REL_OFFSET rdx, 0
> ..
> Ugh.
>
> This is too ugly, it needs to die.
>
> Why doesn't pushq_cfi do that CFI_REL_OFFSET thing?

I don't know either... it seems to be used after push_cfi
*sometimes*, not always:

.macro op_safe_regs op
ENTRY(\op\()_safe_regs)
        CFI_STARTPROC
        pushl_cfi %ebx
        pushl_cfi %ebp
        pushl_cfi %esi
        pushl_cfi %edi

but

ENTRY(csum_partial)
        CFI_STARTPROC
        pushl_cfi %esi
        CFI_REL_OFFSET esi, 0
        pushl_cfi %ebx
        CFI_REL_OFFSET ebx, 0

We need an expert to enlighten us.