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Date:   Thu, 30 Aug 2018 16:58:56 +0300
From:   "Kirill A. Shutemov" <kirill@shutemov.name>
To:     Baoquan He <bhe@redhat.com>
Cc:     tglx@linutronix.de, mingo@kernel.org, hpa@zytor.com,
        kirill.shutemov@linux.intel.com, x86@kernel.org,
        linux-kernel@vger.kernel.org, kexec@lists.infradead.org
Subject: Re: [PATCH 0/3] Add restrictions for kexec/kdump jumping between
 5-level and 4-level kernel
Message-ID: <20180830135855.rylamc7mx2ur3tab@kshutemo-mobl1>
References: <20180829141624.13985-1-bhe@redhat.com>
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On Wed, Aug 29, 2018 at 10:16:21PM +0800, Baoquan He wrote:
> This was suggested by Kirill several months ago, I worked out several
> patches to fix, then interrupted by other issues. So sort them out
> now and post for reviewing.

Thanks for doing this.

> The current upstream kernel supports 5-level paging mode and supports
> dynamically choosing paging mode during bootup according to kernel
> image, hardware and kernel parameter setting. This flexibility brings
> several issues for kexec/kdump:
> 1)
> Switching between paging modes, requires changes into target kernel.
> It means you cannot kexec() 4-level paging kernel from 5-level paging
> kernel if 4-level paging kernel doesn't include changes. 
> 
> 2)
> Switching from 5-level paging to 4-level paging kernel would fail, if
> kexec() put kernel image above 64TiB of memory.

I'm not entirely sure that 64TiB is the limit here. Technically, 4-level
paging allows to address 256TiB in 1-to-1 mapping. We just don't have
machines with that wide physical address space (which don't support
5-level paging too).

What is your reasoning about 64TiB limit?

-- 
 Kirill A. Shutemov