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

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Date:   Wed, 2 Nov 2022 13:57:14 -0500
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 13:49, Borislav Petkov wrote:
> On Wed, Nov 02, 2022 at 11:54:08AM -0500, Eric DeVolder wrote:
>> Technically the answer is no; cpu hotplug events are independent of memory
>> hotplug events, but both are written into the elfcorehdr, so in reality
>> yes... The elfcorehdr contains a single list of Phdrs describing CPUs and
>> crash memory ranges; the entire list is re-written on a hotplug change.
> 
> Then technically also yes. Otherwise your crash information will contain
> wrong CPU numbers.
> 
> How has that not been a problem until now...?
> 
> I.e., offline a bunch of CPUs and then cause a crash dump.
> 
> Hmm.
> 

There is a solution for updating the elfcorehdr today, for when say a bunch of CPUs are offlined. It 
is done via userspace udev rules to do a unload-then-reload of the entire crash kernel system 
(kernel, initrd, purgatory, boot_params, and of course elfcorehdr). This performs extremely poorly 
in highly dynamic hotplug situations (such as when adding alot of memory to a vm), and thus the 
attempt at this solution.

But I sense I missing your point?

Thanks!
eric