linux-kernel - Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

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Date:   Tue, 25 Oct 2022 12:31:59 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Baoquan He <bhe@...hat.com>
Cc:     Eric DeVolder <eric.devolder@...cle.com>,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>, david@...hat.com,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...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,
        sourabhjain@...ux.ibm.com, linux-mm@...ck.org
Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

On Thu, Oct 13, 2022 at 10:57:28AM +0800, Baoquan He wrote:
> The concern to range number mainly is on Virt guest systems.

And why would virt emulate 1K hotpluggable DIMM slots and not emulate a
real machine?

> On baremetal system, basically only very high end server support
> memory hotplug. I ever visited customer's lab and saw one server,
> it owns 8 slots, on each slot a box containing about 20 cpus and 2T
> memory at most can be plugged in at one time. So people won't make too
> many slots for hotplugging since it's too expensive.

There you have it - the persuading argument.

> I checked user space kexec code, the maximum memory range number is
> honored to x86_64 because of a HPE SGI system. After that, nobody
> complains about it. Please see below user space kexec-tools commit in
> https://git.kernel.org/pub/scm/utils/kernel/kexec/kexec-tools.git
> 
> The memory ranges may be not all made by different DIMM slots, could be
> firmware reservatoin, e.g efi/BIOS diggged out physical memory,
			    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

I don't know what that means.

If it is firmware crap, you want to exclude that from kdump anyway.

> Now CONFIG_NR_CPUS has the maximum number as 8192. And user space
> kexec-tools has maximum memory range number as 2048. We can take
> the current 8192 + 2048  = 10K as default value conservatively. Or
> take 8192 + 2048 * 2 = 12K which has two times of maximum memory range
> bumber in kexec-tools. What do you think?

I still think that we should stick to reality and support what is
possible not what is potentially and theoretically there.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette