linux-kernel - Re: [PATCH v13 7/7] x86/crash: add x86 crash hotplug support

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Date:   Wed, 9 Nov 2022 09:48:33 -0600
From:   Eric DeVolder <eric.devolder@...cle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, bhe@...hat.com, vgoyal@...hat.com,
        tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
        hpa@...or.com, nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org, david@...hat.com,
        sourabhjain@...ux.ibm.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v13 7/7] x86/crash: add x86 crash hotplug support

On 11/2/22 04:26, Borislav Petkov wrote:
> On Tue, Nov 01, 2022 at 10:45:00AM -0500, Eric DeVolder wrote:
>> As I'm re-reading that message, I suspect now the preference is to just to
>> strike this ifdiffery line in this file and have the code always present?
>>
>> If the preference is actually for CRASH_HOTPLUG, then let me know.
> 
> Well, it is this part:
> 
> "But on a plain simple laptop or workstation which has CPU hotplug,
> would it make sense for the crash ranges to get updated too when CPUs
> are offlined?
> 
> If so, I think you want this code present there too, without a Kconfig
> item."
> 
> IOW, if this thing doesn't make sense to have on the majority of
> machines out there - and memory hotplug machines are not the majority -
> then it should be behind a Kconfig item which is default off and gets
> enabled only when the user selects crash and memory hotplug...
> 
> I'd say.
> 
Boris,
I apologize for the delay in responding, I've been away for the past week.

I'm re-reading the thread on this topic, and I apologize for backing up a bit,
but as I read the last paragraph again, your contention is that the bit of
code in this file *should* be behind a Kconfig item, and default to off, as
"memory hotplug machines are not the majority".

Does this mean then that I need to introduce CRASH_HOTPLUG again, so it can be
default off?

Examining arch/x86/configs/x86_64_defconfig, neither HOTPLUG_CPU or MEMORY_HOTPLUG are present, but 
CONFIG_SMP=y.

And in examining arch/x86/Kconfig, I see:

config HOTPLUG_CPU
     def_bool y
     depends on SMP

which then defaults HOTPLUG_CPU to on and thus this code/ifdef in question.

And as a reminder, the '#if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)' shows up 
in only three locations: include/linux/kexec.h,  kernel/crash_core.c and arch/x86/kernel/crash.c.

And we resolved in this thread last week that this patch series is useful for cpu and/or memory hotplug.

So at this point, I'm still not sure if you want the ifdef line:
  - removed altogether
  - transitioned to CRASH_HOTPLUG
  - leave as is

If I could get clarity on that, that would be much appreciated!
Thanks!
eric