From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 4BD01C433F4 for ; Thu, 30 Aug 2018 14:57:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id ED0C32073D for ; Thu, 30 Aug 2018 14:57:58 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org ED0C32073D Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=redhat.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728630AbeH3TA3 (ORCPT ); Thu, 30 Aug 2018 15:00:29 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:60242 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1728449AbeH3TA3 (ORCPT ); Thu, 30 Aug 2018 15:00:29 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 4EA7FB5CB; Thu, 30 Aug 2018 14:57:55 +0000 (UTC) Received: from localhost (ovpn-8-29.pek2.redhat.com [10.72.8.29]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 8D13A9465E; Thu, 30 Aug 2018 14:57:54 +0000 (UTC) Date: Thu, 30 Aug 2018 22:57:51 +0800 From: Baoquan He To: "Kirill A. Shutemov" Cc: "Kirill A. Shutemov" , tglx@linutronix.de, mingo@kernel.org, hpa@zytor.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: <20180830145751.GC14702@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180830142739.gfpa23nvex7xbkkf@black.fi.intel.com> User-Agent: Mutt/1.9.1 (2017-09-22) X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.1]); Thu, 30 Aug 2018 14:57:55 +0000 (UTC) X-Greylist: inspected by milter-greylist-4.5.16 (mx1.redhat.com [10.11.55.1]); Thu, 30 Aug 2018 14:57:55 +0000 (UTC) for IP:'10.11.54.5' DOMAIN:'int-mx05.intmail.prod.int.rdu2.redhat.com' HELO:'smtp.corp.redhat.com' FROM:'bhe@redhat.com' RCPT:'' Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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