From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Vrabel Subject: Re: dom0 pvops and rearranging memory layout Date: Fri, 23 Jan 2015 11:58:23 +0000 Message-ID: <54C2375F.4050607@citrix.com> References: <54C22334.2070604@suse.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <54C22334.2070604@suse.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Juergen Gross , "xen-devel@lists.xensource.com" , Konrad Rzeszutek Wilk , Boris Ostrovsky Cc: Jan Beulich List-Id: xen-devel@lists.xenproject.org On 23/01/15 10:32, Juergen Gross wrote: > Hi, > > while testing new patches to support dom0 with more than 512 GB I > stumbled over an issue which - I think - is present in pvops for > some time now. > > On boot the kernel rearranges the memory layout to match the host > E820 map. This is done to be able to access all I/O areas with > identity mapped pfns (pfn == mfn). So basically some memory pages > change their pfns while the mfns stay the same. > > There is no check done whether the moved memory areas are actually > in use (e.g. via memblock_is_reserved()). This can lead to cases > where memory in use is put to an area which is made available for > new memory allocations soon afterwards. Memory in question could > be the initrd, the p2m map presented to dom0 by the hypervisor, or > (hopefully in theory only) even the kernel itself or it's initial > page tables built by the hypervisor. > > In my test I had a p2m map of nearly 2GB size and the area between > 2GB and 4GB had no RAM. So parts of the p2m map and the complete > initrd where subject to be remapped which led to an early PANIC. > > I'll try to add some special handling for the initrd and the p2m > map. In case someone has a better idea: please tell me. I would suggest: Pass 1: relocate p2m and page tables to "safe" areas. Pass 2: relocate frames from holes/reserved regions. I don't think we want to change the hypervisor to workaround this. David