linux-kernel - Re: [PATCH v3 1/4] DYNAMIC_FTRACE configurable with and without REGS

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Message-ID: <CAKv+Gu-ZtJ-BpfPWU=soDCb2+dAd81p8OtaarWhmTe+Fk6A8ig@mail.gmail.com>
Date:   Mon, 1 Oct 2018 16:52:27 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Torsten Duwe <duwe@....de>
Cc:     Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
        Will Deacon <will.deacon@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Julien Thierry <julien.thierry@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        live-patching@...r.kernel.org
Subject: Re: [PATCH v3 1/4] DYNAMIC_FTRACE configurable with and without REGS

Hi Torsten,

On 1 October 2018 at 16:16, Torsten Duwe <duwe@....de> wrote:
> In commit 06aeaaeabf69da4, many ftrace-related config options are
> consolidated. By accident, I guess, the choice about DYNAMIC_FTRACE
> and DYNAMIC_FTRACE_WITH_REGS is no longer available explicitly but
> determined by the sole availability on the architecture.
>
> This makes it hard to introduce DYNAMIC_FTRACE_WITH_REGS if it depends
> on new compiler features or other new properties of the toolchain
> without breaking existing configurations.
>
> This patch turns the def_bool into an actual choice. Should the toolchain
> not meet the requirements for _WITH_REGS it can be turned off.
>

I guess we now have Kbuild/Kconfig support for this, no? I mean, we
can now show/hide options depending on the capabilities of the
toolchain.

I am not saying it would be a better approach, though - I'd rather
have a warning than have things silently disabled, but other people
may think differently.


> Signed-off-by: Torsten Duwe <duwe@...e.de>
>
>
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -508,9 +508,15 @@ config DYNAMIC_FTRACE
>           otherwise has native performance as long as no tracing is active.
>
>  config DYNAMIC_FTRACE_WITH_REGS
> -       def_bool y
> +       bool "Include register content tracking in dynamic ftrace facility"
> +       default y
>         depends on DYNAMIC_FTRACE
>         depends on HAVE_DYNAMIC_FTRACE_WITH_REGS
> +       help
> +         This architecture supports the inspection of register contents,
> +         as passed between functions, at the dynamic ftrace points.
> +         This is also a prerequisite for Kernel Live Patching (KLP).
> +         When in doubt, say Y.
>
>  config FUNCTION_PROFILER
>         bool "Kernel function profiler"