Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

From: Sourabh Jain <sourabhjain@linux.ibm.com>
To: Eric DeVolder <eric.devolder@oracle.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
Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support
Date: Tue, 4 Oct 2022 12:33:24 +0530	[thread overview]
Message-ID: <a2e0b554-2ef4-1266-cc3a-fa5a28049131@linux.ibm.com> (raw)
In-Reply-To: <fd08c13d-a917-4cd6-85ec-267e0fe74c41@oracle.com>

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