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Date:   Wed, 12 Oct 2022 15:19:19 -0500
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Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support
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To:     Borislav Petkov <bp@alien8.de>, Baoquan He <bhe@redhat.com>
Cc:     Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>, david@redhat.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, 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,
        sourabhjain@linux.ibm.com, linux-mm@kvack.org
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On 10/12/22 12:46, Borislav Petkov wrote:
> On Sat, Oct 08, 2022 at 10:35:14AM +0800, Baoquan He wrote:
>> Memory hptlug is not limited by a certin or a max number of memory
>> regions, but limited by how large of the linear mapping range which
>> physical can be mapped into.
> 
> Memory hotplug is not limited by some abstract range but by the *actual*
> possibility of how many DIMM slots on any motherboard can hotplug
> memory. Certainly not 32K.
> 
> So you can choose a sane default which covers *all* actual systems out
> there.


We run here QEMU with the ability for 1024 DIMM slots. A DIMM can be any
reasonable power of 2 size, and then that DIMM is further divided into memblocks,
typically 128MiB.

So, for example, 1TiB requires 1024 DIMMs of 1GiB each with 128MiB memblocks, that results
in 8K possible memory regions. So just going to 4TiB reaches 32K memory regions.

This I can attest for virtualized DIMMs, not sure about other memory hotplug technologies
like virtio-mem or DynamicMemory. But it seems reasonable that those technologies could
also easily reach into these number ranges.

Eric

> 
>> For the Kconfig CRASH_MAX_MEMORY_RANGES Eric added, it's meaningful to
>> me to set a fixed value which is enough in reality.
> 
> Yes, exactly.
> 
>> For extreme testing with special purpose, it could be broken easily,
>> people need decide by self whether the CONFIG_CRASH_MAX_MEMORY_RANGES
>> is enlarged or not.
> 
> I don't want for people to decide on one more thing where they have to
> go and read a bunch of specs just to know what is a good value. So we
> should set a sane, *practical* upper limit and simply go with it.
> 
> Everything else is testing stuff and if you test the kernel, then you
> can change limits and values and so on as you want to.
> 
> Thx.
>