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

From: Alexandre Ghiti <alexandre.ghiti@canonical.com>
To: Baoquan He <bhe@redhat.com>
Cc: Jisheng Zhang <jszhang@kernel.org>,
	Russell King <linux@armlinux.org.uk>,
	 Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	 Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	 Albert Ou <aou@eecs.berkeley.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	 Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	 Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, hpa@zytor.com,
	 Eric Biederman <ebiederm@xmission.com>,
	linux-arm-kernel@lists.infradead.org,
	 linux-kernel@vger.kernel.org, linux-riscv@lists.infradead.org,
	 kexec@lists.infradead.org, Alexandre ghiti <alex@ghiti.fr>
Subject: Re: [PATCH v2 0/5] kexec: use IS_ENABLED(CONFIG_KEXEC_CORE) instead of #ifdef
Date: Wed, 19 Jan 2022 09:52:16 +0100	[thread overview]
Message-ID: <CA+zEjCvqYAWaWW9hVbHqYm_GbV-BaiOM4c77WHnxDNh+SRDUuw@mail.gmail.com> (raw)
In-Reply-To: <20220119080859.GB4977@MiWiFi-R3L-srv>

Hi Baoquan,

On Wed, Jan 19, 2022 at 9:11 AM Baoquan He <bhe@redhat.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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