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Date:   Sat, 29 Apr 2023 00:01:03 +0530
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Subject: Re: [PATCH v21 5/7] x86/crash: add x86 crash hotplug support
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To:     Baoquan He <bhe@redhat.com>
Cc:     Eric DeVolder <eric.devolder@oracle.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
        dyoung@redhat.com, vgoyal@redhat.com, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        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: <20230404180326.6890-1-eric.devolder@oracle.com>
 <20230404180326.6890-6-eric.devolder@oracle.com>
 <da95e01d-24bd-676b-3a48-bafda1b36cda@linux.ibm.com>
 <ZEo3NK/9Nkc7ctrc@bhe.users.ipa.redhat.com>
 <409c8253-49b9-6993-f79e-8e6203ce4b31@linux.ibm.com>
 <ZEuQ/hxmZey+eFZs@MiWiFi-R3L-srv>
From:   Hari Bathini <hbathini@linux.ibm.com>
In-Reply-To: <ZEuQ/hxmZey+eFZs@MiWiFi-R3L-srv>
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On 28/04/23 2:55 pm, Baoquan He wrote:
> On 04/27/23 at 10:26pm, Hari Bathini wrote:
>> 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.
> 
> Yeah, sounds like a good idea. It may be extended in later patch.

Fixing it in this series itself would be a cleaner way, I guess.

Thanks
Hari