From mboxrd@z Thu Jan 1 00:00:00 1970 From: Juergen Gross Subject: Re: dom0 pvops and rearranging memory layout Date: Fri, 23 Jan 2015 12:41:20 +0100 Message-ID: <54C23360.4040603@suse.com> References: <54C22334.2070604@suse.com> <54C2321A.7020402@citrix.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii"; Format="flowed" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <54C2321A.7020402@citrix.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: Andrew Cooper , "xen-devel@lists.xensource.com" , David Vrabel , Konrad Rzeszutek Wilk , Boris Ostrovsky Cc: Jan Beulich List-Id: xen-devel@lists.xenproject.org On 01/23/2015 12:35 PM, Andrew Cooper wrote: > 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. >> > > The relocation is done based only on the e820 is it not? Yes. > I wonder whether it might be reasonable to extend contruct_dom0/libelf > to avoid constructing a p2m where pfns of built data (kernel, initrd, > p2m and initial pagetables) aliased with host io regions. That was my first idea, too. OTOH this would require a rather new hypervisor with this functionality to be able to run a pvops dom0 on such a machine. Ans can we be sure that an existing non-pvops dom0 (or even an old pvops one) can work with such a change? Juergen