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

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Date:   Thu, 27 Apr 2023 22:26:54 +0530
From:   Hari Bathini <hbathini@...ux.ibm.com>
To:     Baoquan He <bhe@...hat.com>
Cc:     Eric DeVolder <eric.devolder@...cle.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...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,
        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

On 27/04/23 2:19 pm, Baoquan He wrote:
> On 04/27/23 at 12:39pm, Hari Bathini wrote:
>> 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.
> 
> Eric has posted patchset to modify kexec_tools to support that, please
> see the link Eric pasted in the cover letter.
> 
> http://lists.infradead.org/pipermail/kexec/2022-October/026032.html

Right, Baoquan.

I did see that and if I read the code correctly, without that patchset
kexec_load would fail. Not with an explicit error that hotplug support
is missing or such but it would simply fail to boot into capture kernel
with digest verification failure.

My suggestion was to avoid that userspace tool breakage for older
kexec-tools version by introducing a new kexec flag that can tell
kernel that kexec-tools is ready to use this in-kernel update support.
So, if kexec_load happens without the flag, avoid doing an in-kernel
update on hotplug. I hope that clears the confusion.

Thanks
Hari