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Date:   Mon, 16 Jan 2023 17:30:05 +0100
From:   Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To:     Eric DeVolder <eric.devolder@oracle.com>
Cc:     Sourabh Jain <sourabhjain@linux.ibm.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
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        robh@kernel.org, efault@gmx.de, rppt@kernel.org, david@redhat.com,
        konrad.wilk@oracle.com, boris.ostrovsky@oracle.com
Subject: Re: [PATCH v15 1/7] crash: move crash_prepare_elf64_headers()
Message-ID: <20230116173005.21bd7b67@meshulam.tesarici.cz>
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        <09567e13-c5ed-d1b9-027c-9340fce6a0a8@linux.ibm.com>
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Hi Eric,

On Thu, 12 Jan 2023 11:23:18 -0600
Eric DeVolder <eric.devolder@oracle.com> wrote:

>[...]
> After looking into this for a bit, to allow hotplug without kexec_file would require quite a bit of 
> code movement. Why? Because hotplug is basically built on top of (part of) the infrastructure that 
> was needed for kexec_file.
> 
> I'd be inclined to suggest that KEXEC_FILE be a required dependency for CRASH_HOTPLUG, ie:
> 
>   config CRASH_HOTPLUG
>          bool "Update the crash elfcorehdr on system configuration changes"
>          default n
> -       depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG)
> +       depends on CRASH_DUMP && KEXEC_FILE && (HOTPLUG_CPU || MEMORY_HOTPLUG)
> 
> 
> If that isn't feasible, then it would appear quite a bit of surgery is needed to properly separate 
> out the items hotplug needs from kexec_file.
> 
> Thoughts?

I would have thought that CPU hotplug can be handled in the kernel only
if the crash image was loaded by the kernel with kexec_file_load(2).
When the image is loaded with kexec_load(2), then all data structures
are prepared by the user-space utility kexec(8), and the kernel
generally has no idea how to handle them.

In short, I believe that by definition there must be this dependency of
CRASH_HOTPLUG on KEXEC_FILE.

Petr T