linux-kernel - Re: [PATCH v2 0/5] kexec: use IS_ENABLED(CONFIG_KEXEC

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Date:   Wed, 19 Jan 2022 16:08:59 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Jisheng Zhang <jszhang@...nel.org>
Cc:     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

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?