linux-kernel - Re: [PATCH] ARM: use choice for kernel unwinders

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Message-ID: <CAK8P3a0oaMS12R4tazChMHENQduO0-Wb-gw-inKjH0oz7_5s3g@mail.gmail.com>
Date:   Wed, 22 Aug 2018 12:02:19 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Stefan Agner <stefan@...er.ch>
Cc:     Russell King - ARM Linux <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ARM: use choice for kernel unwinders

On Wed, Aug 22, 2018 at 12:24 AM Stefan Agner <stefan@...er.ch> wrote:
>
> While in theory multiple unwinders could be compiled in, it does
> not make sense in practise. Use a choice to make the unwinder
> selection mutually exclusive and mandatory.
>
> Already before this commit it has not been possible to deselect
> FRAME_POINTER. Remove the obsolete comment.
>
> Furthermore, to produce a meaningful backtrace with FRAME_POINTER
> enabled the kernel needs a specific function prologue:
>     mov    ip, sp
>     stmfd    sp!, {fp, ip, lr, pc}
>     sub    fp, ip, #4
>
> To get to the required prologue gcc uses apcs and no-sched-prolog.
> This compiler options are not available on clang, and clang is not
> able to generate the required prologue. Make the FRAME_POINTER
> config symbol depending on !clang.
>
> Suggested-by: Arnd Bergmann <arnd@...db.de>
> Signed-off-by: Stefan Agner <stefan@...er.ch>

Looks ok to me. I've added it to my randconfig test environment, you
will hear from me within a day if I run into build regressions.

We may still want to clean up these three lines:

lib/Kconfig.debug:  select FRAME_POINTER if !MIPS && !PPC &&
!ARM_UNWIND && !S390 && !MICROBLAZE && !ARC && !X86
lib/Kconfig.debug:  select FRAME_POINTER if !MIPS && !PPC && !S390 &&
!MICROBLAZE && !ARM_UNWIND && !ARC && !X86
lib/Kconfig.debug:  select FRAME_POINTER if !MIPS && !PPC && !S390 &&
!MICROBLAZE && !ARM_UNWIND && !ARC && !X86

in which ARM is the odd case that currently depends on an architecture
specific rather than the architecture itself.
We could introduce a 'config ARCH_HAS_UNWINDER' symbol that gets
selected by mips, ppc, s390, microblaze, arm and x86 unconditionally,
and then simplify the 'select' statements here.

       Arnd