linux-kernel - Re: [PATCH v21 5/7] x86/crash: add x86 crash hotplug support

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Date:   Thu, 27 Apr 2023 12:39:03 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     Eric DeVolder <eric.devolder@...cle.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
Cc:     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,
        sourabhjain@...ux.ibm.com, konrad.wilk@...cle.com,
        boris.ostrovsky@...cle.com
Subject: Re: [PATCH v21 5/7] x86/crash: add x86 crash hotplug support

Hi Eric,

On 04/04/23 11:33 pm, Eric DeVolder wrote:
> When CPU or memory is hot un/plugged, or off/onlined, the crash
> elfcorehdr, which describes the CPUs and memory in the system,
> must also be updated.
> 
> The segment containing the elfcorehdr is identified at run-time
> in crash_core:crash_handle_hotplug_event(), which works for both
> the kexec_load() and kexec_file_load() syscalls. A new elfcorehdr
> is generated from the available CPUs and memory into a buffer,
> and then installed over the top of the existing elfcorehdr.
> 
> In the patch 'kexec: exclude elfcorehdr from the segment digest'
> the need to update purgatory due to the change in elfcorehdr was
> eliminated.  As a result, no changes to purgatory or boot_params
> (as the elfcorehdr= kernel command line parameter pointer
> remains unchanged and correct) are needed, just elfcorehdr.
> 
> To accommodate a growing number of resources via hotplug, the
> elfcorehdr segment must be sufficiently large enough to accommodate
> changes, see the CRASH_MAX_MEMORY_RANGES description. This is used
> only on the kexec_file_load() syscall; for kexec_load() userspace
> will need to size the segment similarly.
> 
> To accommodate kexec_load() syscall in the absence of

Firstly, thanks! This series is a nice improvement to kdump support
in hotplug environment.

One concern though is that this change assumes corresponding support
in kexec-tools. Without that support kexec_load would fail to boot
with digest verification failure, iiuc.

I would suggest a flag to advertise to the kernel that kexec-tools/
userspace wants in-kernel update. Something like KEXEC_IN_KERNEL_UPDATE
on top of existing flags like KEXEC_ON_CRASH & KEXEC_PRESERVE_CONTEXT.
This flag can be used to decide whether in-kernel update needs to be
enforced or not. That should make transition to this change smoother
without having to break userspace.

Thanks
Hari