linux-kernel - Re: [PATCH v15 1/7] crash: move crash_prepare_elf64

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Message-ID: <20230116173005.21bd7b67@meshulam.tesarici.cz>
Date:   Mon, 16 Jan 2023 17:30:05 +0100
From:   Petr Tesařík <petr@...arici.cz>
To:     Eric DeVolder <eric.devolder@...cle.com>
Cc:     Sourabh Jain <sourabhjain@...ux.ibm.com>,
        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, bp@...en8.de,
        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,
        konrad.wilk@...cle.com, boris.ostrovsky@...cle.com
Subject: Re: [PATCH v15 1/7] crash: move crash_prepare_elf64_headers()

Hi Eric,

On Thu, 12 Jan 2023 11:23:18 -0600
Eric DeVolder <eric.devolder@...cle.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