From mboxrd@z Thu Jan 1 00:00:00 1970 From: Juergen Gross Subject: Re: [Xen-devel] [PATCH 3/3] xen: eliminate scalability issues from initial mapping setup Date: Fri, 05 Sep 2014 09:55:47 +0200 Message-ID: <54096C83.2080608@suse.com> References: <1409834326-29287-1-git-send-email-jgross@suse.com> <1409834326-29287-4-git-send-email-jgross@suse.com> <54086244.7020601@citrix.com> <540862FE.7070108@citrix.com> <540893F00200007800030E4F@mail.emea.novell.com> <54087AA1.8010208@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <54087AA1.8010208@citrix.com> Sender: linux-kernel-owner@vger.kernel.org To: Andrew Cooper , Jan Beulich Cc: David Vrabel , xen-devel@lists.xensource.com, boris.ostrovsky@oracle.com, konrad.wilk@oracle.com, linux-kernel@vger.kernel.org List-Id: xen-devel@lists.xenproject.org On 09/04/2014 04:43 PM, Andrew Cooper wrote: > On 04/09/14 15:31, Jan Beulich wrote: >>>>> On 04.09.14 at 15:02, wrote: >>> On 04/09/14 13:59, David Vrabel wrote: >>>> On 04/09/14 13:38, Juergen Gross wrote: >>>>> Direct Xen to place the initial P->M table outside of the initial >>>>> mapping, as otherwise the 1G (implementation) / 2G (theoretical) >>>>> restriction on the size of the initial mapping limits the amount >>>>> of memory a domain can be handed initially. >>>> The three level p2m limits memory to 512 GiB on x86-64 but this patch >>>> doesn't seem to address this limit and thus seems a bit useless to me. >>> Any increase of the p2m beyond 3 levels will need to come with >>> substantial libxc changes first. 3 level p2ms are hard coded throughout >>> all the PV build and migrate code. >> No, there no such dependency - the kernel could use 4 levels at >> any time (sacrificing being able to get migrated), making sure it >> only exposes the 3 levels hanging off the fourth level (or not >> exposing this information at all) to external entities making this >> wrong assumption. >> >> Jan >> > > That would require that the PV kernel must start with a 3 level p2m and > fudge things afterwards. I always thought the 3 level p2m is constructed by the kernel, not by the tools. It starts with the linear p2m list anchored at xen_start_info->mfn_list, constructs the p2m tree and writes the p2m_top_mfn mfn to HYPERVISOR_shared_info->arch.pfn_to_mfn_frame_list_list See comment in the kernel source arch/x86/xen/p2m.c So booting with a larger p2m list can be handled completely by the kernel itself. > > At a minimum, I would expect a patch to libxc to detect a 4 level PV > guest and fail with a meaningful error, rather than an obscure "m2p > doesn't match p2m for mfn/pfn X". I'd rather fix it in a clean way. I think the best way to do it would be an indicator in the p2m array anchor, e.g. setting 1<<61 in pfn_to_mfn_frame_list_list. This will result in an early error with old tools: "Couldn't map p2m_frame_list_list" Juergen