linux-kernel - Re: [PATCH v2 0/5] kexec: use IS_ENABLED(CONFIG_KEXEC_CORE) instead of #ifdef

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Date:   Wed, 19 Jan 2022 09:52:16 +0100
From:   Alexandre Ghiti <alexandre.ghiti@...onical.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Jisheng Zhang <jszhang@...nel.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        hpa@...or.com, Eric Biederman <ebiederm@...ssion.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        linux-riscv@...ts.infradead.org, kexec@...ts.infradead.org,
        Alexandre ghiti <alex@...ti.fr>
Subject: Re: [PATCH v2 0/5] kexec: use IS_ENABLED(CONFIG_KEXEC_CORE) instead
 of #ifdef

Hi Baoquan,

On Wed, Jan 19, 2022 at 9:11 AM Baoquan He <bhe@...hat.com> wrote:
>
> On 01/18/22 at 10:13pm, Jisheng Zhang wrote:
> > On Sun, Jan 16, 2022 at 09:38:47PM +0800, Baoquan He wrote:
> > > Hi Jisheng,
> >
> > Hi Baoquan,
> >
> > >
> > > On 12/07/21 at 12:05am, Jisheng Zhang wrote:
> > > > Replace the conditional compilation using "#ifdef CONFIG_KEXEC_CORE"
> > > > by a check for "IS_ENABLED(CONFIG_KEXEC_CORE)", to simplify the code
> > > > and increase compile coverage.
> > >
> > > I go through this patchset, You mention the benefits it brings are
> > > 1) simplity the code;
> > > 2) increase compile coverage;
> > >
> > > For benefit 1), it mainly removes the dummy function in x86, arm and
> > > arm64, right?
> >
> > Another benefit: remove those #ifdef #else #endif usage. Recently, I
> > fixed a bug due to lots of "#ifdefs":
> > http://lists.infradead.org/pipermail/linux-riscv/2021-December/010607.html
>
> Glad to know the fix. While, sometime the ifdeffery is necessary. I am
> sorry about the one in riscv and you have fixed, it's truly a bug . But,
> the increasing compile coverage at below you tried to make, it may cause
> issue. Please see below my comment.
>
> >
> > >
> > > For benefit 2), increasing compile coverage, could you tell more how it
> > > achieves and why it matters? What if people disables CONFIG_KEXEC_CORE in
> > > purpose? Please forgive my poor compiling knowledge.
> >
> > Just my humble opinion, let's compare the code::
> >
> > #ifdef CONFIG_KEXEC_CORE
> >
> > code block A;
> >
> > #endif
> >
> > If KEXEC_CORE is disabled, code block A won't be compiled at all, the
> > preprocessor will remove code block A;
> >
> > If we convert the code to:
> >
> > if (IS_ENABLED(CONFIG_KEXEC_CORE)) {
> >       code block A;
> > }
> >
> > Even if KEXEC_CORE is disabled, code block A is still compiled.
>
> This is what I am worried about. Before, if CONFIG_KEXEC_CORE is
> unset, those relevant codes are not compiled in. I can't see what
> benefit is brought in if compiled in the unneeded code block. Do I miss
> anything?
>

This is explained in Documentation/process/coding-style.rst "21)
Conditional Compilation".

Alex

>
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