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Date:   Fri, 7 Oct 2022 14:56:00 -0500
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Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support
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To:     Sourabh Jain <sourabhjain@linux.ibm.com>,
        Borislav Petkov <bp@alien8.de>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
        dyoung@redhat.com, bhe@redhat.com, vgoyal@redhat.com,
        tglx@linutronix.de, mingo@redhat.com, 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,
        konrad.wilk@oracle.com, boris.ostrovsky@oracle.com
References: <20220909210509.6286-1-eric.devolder@oracle.com>
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From:   Eric DeVolder <eric.devolder@oracle.com>
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On 10/4/22 02:03, Sourabh Jain wrote:
> 
> On 30/09/22 21:06, Eric DeVolder wrote:
>>
>>
>> On 9/28/22 11:07, Borislav Petkov wrote:
>>> On Tue, Sep 13, 2022 at 02:12:31PM -0500, Eric DeVolder wrote:
>>>> This topic was discussed previously https://lkml.org/lkml/2022/3/3/372.
>>>
>>> Please do not use lkml.org to refer to lkml messages. We have a
>>> perfectly fine archival system at lore.kernel.org. You simply do
>>>
>>> https://lore.kernel.org/r/<Message-ID>
>>>
>>> when you want to point to a previous mail.
>>
>> ok, thanks for pointing that out to me.
>>>
>>>> David points out that terminology is tricky here due to differing behaviors.
>>>> And perhaps that is your point in asking for guidance text. It can be
>>>> complicated
>>>
>>> Which means you need an explanation how to use this even more.
>>>
>>> And why is CONFIG_CRASH_MAX_MEMORY_RANGES even a Kconfig item and not
>>> something you discover from the hardware?
>>
>> No, is the short answer.
>>
>>>
>>> Your help text talks about System RAM entries in /proc/iomem which means
>>> that those entries are present somewhere in the kernel and you can read
>>> them out and do the proper calculations dynamically instead of doing the
>>> static CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES thing.
>>
>> The intent is to compute the max size buffer needed to contain a maximum populated elfcorehdr, 
>> which is primarily based on the number of CPUs and memory regions. Thus far I (and others 
>> involved) have not found a kernel method to determine the maximum number of memory regions 
>> possible (if you are aware of one, please let me know!). Thus CONFIG_CRASH_MAX_MEMORY_RANGES was 
>> born (rather borrowed from kexec-tools).
>>
>> So no dynamic computation is possible, yet.
> 
> Hello Eric,
> 
> How about allocating buffer space for max program header possible in a elfcorehdr?
> 
> mage->elf_headers_sz = kbuf.memsz = PN_XNUM * sizeof(Elf64_Phdr);
> 
> PN_XNUM is part of linux/elf.h (include/uapi/linux/elf.h).
> 
> Refer below link for more details:
> https://man7.org/linux/man-pages/man5/elf.5.html
> 
> Thanks,
> Sourabh Jain
> 

Well, that is an idea. I'm not sure it is the answer yet, but if I do compute
a value, then that value needs to be checked against PN_XNUM so it still results
in a valid elfcorehdr.

Thanks,
eric