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

From: Eric DeVolder <eric.devolder@oracle.com>
To: Borislav Petkov <bp@alien8.de>
Cc: linux-kernel@vger.kernel.org, x86@kernel.org,
	kexec@lists.infradead.org, ebiederm@xmission.com,
	dyoung@redhat.com, bhe@redhat.com, vgoyal@redhat.com,
	tglx@linutronix.de, mingo@redhat.com,
	dave.hansen@linux.intel.com, hpa@zytor.com,
	nramas@linux.microsoft.com, thomas.lendacky@amd.com,
	robh@kernel.org, efault@gmx.de, rppt@kernel.org,
	david@redhat.com, sourabhjain@linux.ibm.com,
	konrad.wilk@oracle.com, boris.ostrovsky@oracle.com
Subject: Re: [PATCH v13 7/7] x86/crash: add x86 crash hotplug support
Date: Wed, 9 Nov 2022 16:12:24 -0600	[thread overview]
Message-ID: <ad35b9b1-447e-c65e-a083-13a8e36c8a5a@oracle.com> (raw)
In-Reply-To: <Y2wcJOehWcG7w5c4@zn.tnic>


On 11/9/22 15:31, Borislav Petkov wrote:
> On Wed, Nov 09, 2022 at 09:48:33AM -0600, Eric DeVolder wrote:
>> ...
>> which then defaults HOTPLUG_CPU to on and thus this code/ifdef in question.
> 
> defconfig can sometimes lag reality. In this case, the majority of
> machines have SMP=y because the majority of machines out there are,
> well, multicore.
> 
>> So at this point, I'm still not sure if you want the ifdef line:
>>   - removed altogether
>>   - transitioned to CRASH_HOTPLUG
>>   - leave as is
> 
> So let's think out loud:
> 
> * the majority of machines will have CONFIG_HOTPLUG_CPU=y because
> they're SMP machines and we want the elfcorehdr updates to happen when
> CPUs get offlined or onlined.
> 
> CONFIG_MEMORY_HOTPLUG is most likely going to be =n on the majority of
> machines out there.
> 
> (Note how the deciding factor for all this is what would make sense on
> the prevailing majority of machines out there.)
> 
> And memory hotplug will be off for the simple reason that not so many
> machines have memory hotplug hardware capability.
> 
> Which then means, IMHO, this functionality should be separate: have a
> CPU hotplug callback and a memory hotplug callback.
> 
> And you kinda do that in
> 
> Subject: [PATCH v13 3/7] crash: add generic infrastructure for crash hotplug support
> 
> but then this all calls into a single handle_hotplug_event() and that
> hp_action doesn't really matter.
> 
> It is used in the call to
> 
>    arch_crash_handle_hotplug_event(image, hp_action);
> 
> but that hp_action argument is unused in the x86 version. >
> IOW, you can do this callback regardless whether it is a CPU or memory
> hotplug event.
> 
> So thinking about it, a single CONFIG_CRASH_HOTPLUG which unifies those
> CPU and memory hotplug callback functionality makes most sense to me.
> Because you don't really differentiate between the two in the callback
> actions.
> 
> Anyway, this is how I see it from here. I could very well be missing an
> aspect, of course.
> 
> Thx.
> 
OK, I'll put in CRASH_HOTPLUG! Expect v14 soon!
Thank you!
eric

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