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Date:   Wed, 19 Jan 2022 16:08:59 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Jisheng Zhang <jszhang@kernel.org>
Cc:     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
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References: <20211206160514.2000-1-jszhang@kernel.org>
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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?