From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mx3-rdu2.redhat.com ([66.187.233.73] helo=mx1.redhat.com) by bombadil.infradead.org with esmtps (Exim 4.90_1 #2 (Red Hat Linux)) id 1fvORt-0008An-Dd for kexec@lists.infradead.org; Thu, 30 Aug 2018 15:01:31 +0000 Date: Thu, 30 Aug 2018 23:01:11 +0800 From: Baoquan He Subject: Re: [PATCH 0/3] Add restrictions for kexec/kdump jumping between 5-level and 4-level kernel Message-ID: <20180830150035.GD14702@192.168.1.2> References: <20180829141624.13985-1-bhe@redhat.com> <20180830135855.rylamc7mx2ur3tab@kshutemo-mobl1> <20180830141202.GA14702@192.168.1.2> <20180830142739.gfpa23nvex7xbkkf@black.fi.intel.com> <20180830145751.GC14702@192.168.1.2> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20180830145751.GC14702@192.168.1.2> List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "kexec" Errors-To: kexec-bounces+dwmw2=infradead.org@lists.infradead.org To: "Kirill A. Shutemov" Cc: x86@kernel.org, kexec@lists.infradead.org, linux-kernel@vger.kernel.org, hpa@zytor.com, "Kirill A. Shutemov" , tglx@linutronix.de, mingo@kernel.org On 08/30/18 at 10:57pm, Baoquan He wrote: > On 08/30/18 at 05:27pm, Kirill A. Shutemov wrote: > > On Thu, Aug 30, 2018 at 02:12:02PM +0000, Baoquan He wrote: > > > On 08/30/18 at 04:58pm, Kirill A. Shutemov wrote: > > > > 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). > > > > > > Hmm, afaik, the MAX_PHYSMEM_BITS limits the maximum address space > > > which physical RAM can mapped to. We have 256TB for the whole address > > > space for 4-level paging, that includes user space and kernel space, > > > it might not allow 256TB entirely for the direct mapping. > > > And the direct mapping is only for physical RAM mapping, and > > > kexec/kdump only cares about the physical RAM space and load them > > > inside. > > > > > > # define MAX_PHYSMEM_BITS (pgtable_l5_enabled() ? 52 : 46) > > > > > > Not sure if my understanding is right, please correct me if I am wrong. > > > > IIRC, we only care about the place kexec puts the kernel before it gets > > decompressed. After the decompression kernel will be put into the right > > spot. > > > > Decompression is done in early boot where we use 1-to-1 mapping (not a > > usual kernel virtual memory layout). All 256TiB should be reachable. > > My understanding that is although it's 1:1 identity mapping, it still ~is that~ , sorry, typo > has to be inside available physical RAM region. I don't remember what > the old code did, now in __startup_64(), you can see that there's a > check like below, and at this time, it's still identity mapping. > > /* Is the address too large? */ > if (physaddr >> MAX_PHYSMEM_BITS) > for (;;); > > Thanks > Baoquan _______________________________________________ kexec mailing list kexec@lists.infradead.org http://lists.infradead.org/mailman/listinfo/kexec