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Date:   Wed, 2 Nov 2022 13:57:14 -0500
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Subject: Re: [PATCH v13 7/7] x86/crash: add x86 crash hotplug support
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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,
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        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
References: <20221031193604.28779-1-eric.devolder@oracle.com>
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From:   Eric DeVolder <eric.devolder@oracle.com>
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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