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Date:   Wed, 2 Mar 2022 10:20:34 +0100
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Subject: Re: [PATCH v4 02/10] crash hp: Introduce CRASH_HOTPLUG configuration
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To:     Eric DeVolder <eric.devolder@oracle.com>,
        Baoquan He <bhe@redhat.com>
Cc:     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,
        konrad.wilk@oracle.com, boris.ostrovsky@oracle.com
References: <20220209195706.51522-1-eric.devolder@oracle.com>
 <20220209195706.51522-3-eric.devolder@oracle.com>
 <YhWpF/Cj/V+Cx6+d@MiWiFi-R3L-srv>
 <10d67f14-c5fe-0a17-e8b5-97702823cc1c@oracle.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <10d67f14-c5fe-0a17-e8b5-97702823cc1c@oracle.com>
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On 01.03.22 21:04, Eric DeVolder wrote:
> 
> 
> On 2/22/22 21:25, Baoquan He wrote:
>> On 02/09/22 at 02:56pm, Eric DeVolder wrote:
>>> Support for CPU and memory hotplug for crash is controlled by the
>>> CRASH_HOTPLUG configuration option, introduced by this patch.
>>>
>>> The CRASH_HOTPLUG_ELFCOREHDR_SZ related configuration option is
>>> also introduced with this patch.
>>>
>>> Signed-off-by: Eric DeVolder <eric.devolder@oracle.com>
>>> ---
>>>   arch/x86/Kconfig | 26 ++++++++++++++++++++++++++
>>>   1 file changed, 26 insertions(+)
>>>
>>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>>> index ebe8fc76949a..4e3374edab02 100644
>>> --- a/arch/x86/Kconfig
>>> +++ b/arch/x86/Kconfig
>>> @@ -2060,6 +2060,32 @@ config CRASH_DUMP
>>>   	  (CONFIG_RELOCATABLE=y).
>>>   	  For more details see Documentation/admin-guide/kdump/kdump.rst
>>>   
>>> +config CRASH_HOTPLUG
>>> +	bool "kernel updates of crash elfcorehdr"
>>> +	depends on CRASH_DUMP && (HOTPLUG_CPU || MEMORY_HOTPLUG) && KEXEC_FILE
>>> +	help
>>> +	  Enable the kernel to update the crash elfcorehdr (which contains
>>> +	  the list of CPUs and memory regions) directly when hot plug/unplug
>>> +	  of CPUs or memory. Otherwise userspace must monitor these hot
>>> +	  plug/unplug change notifications via udev in order to
>>> +	  unload-then-reload the crash kernel so that the list of CPUs and
>>> +	  memory regions is kept up-to-date. Note that the udev CPU and
>>> +	  memory change notifications still occur (however, userspace is not
>>> +	  required to monitor for crash dump purposes).
>>> +
>>> +config CRASH_HOTPLUG_ELFCOREHDR_SZ
>>> +	depends on CRASH_HOTPLUG
>>> +	int
>>> +	default 131072
>>> +	help
>>> +	  Specify the maximum size of the elfcorehdr buffer/segment.
>>> +	  The 128KiB default is sized so that it can accommodate 2048
>>> +	  Elf64_Phdr, where each Phdr represents either a CPU or a
>>> +	  region of memory.
>>> +	  For example, this size can accommodate hotplugging a machine
>>> +	  with up to 1024 CPUs and up to 1024 memory regions (e.g. 1TiB
>>> +	  with 1024 1GiB memory DIMMs).
>>
>> This example of memory could be a little misleading. The memory regions
>> may not be related to memory DIMMs. System could split them into many
>> smaller regions during bootup.
> 
> I changed "with 1024 1GiB memory DIMMs" to "with 1024 1GiB hotplug memories".
> eric

It's still not quite precise. Essentially it's the individual "System
RAM" entries in /proc/iomem

Boot memory (i.e., a single DIMM) might be represented by multiple
entries due to rearranged holes (by the BIOS).

While hoplugged DIMMs (under virt!) are usually represented using a
single range, it can be different on physical machines. Last but not
least, dax/kmem and virtio-mem behave in a different way.

-- 
Thanks,

David / dhildenb