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Shutemov" To: Baoquan He 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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180829141624.13985-1-bhe@redhat.com> User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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