From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ian Campbell Subject: Re: RE: Ballooning up Date: Tue, 14 Sep 2010 10:07:28 +0100 Message-ID: <1284455248.14311.16824.camel@zakaz.uk.xensource.com> References: <4C85F973.2030007@goop.org> <54eebb3a-f539-43be-8134-a969a4f671c4@default> <4C8EAB0E.7040407@goop.org> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <4C8EAB0E.7040407@goop.org> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Jeremy Fitzhardinge Cc: Dan Magenheimer , "Xen-devel@lists.xensource.com" , Konrad Wilk , Daniel Kiper , Stefano Stabellini List-Id: xen-devel@lists.xenproject.org On Mon, 2010-09-13 at 23:51 +0100, Jeremy Fitzhardinge wrote: > On 09/13/2010 02:17 PM, Dan Magenheimer wrote: > >> As a side-effect, it also works for dom0. If you set dom0_mem on the > >> Xen command line, then nr_pages is limited to that value, but the > >> kernel > >> can still see the system's real E820 map, and therefore adds all the > >> system's memory to its own balloon driver, potentially allowing dom0 to > >> expand up to take all physical memory. > >> > >> However, this may caused bad side-effects if your system memory is much > >> larger than your dom0_mem, especially if you use a 32-bit dom0. I may > >> need to add a kernel command line option to limit the max initial > >> balloon size to mitigate this... > > I would call this dom0 functionality a bug. I think both Citrix > > and Oracle use dom0_mem as a normal boot option for every > > installation and, while I think both employ heuristics to choose > > a larger dom0_mem for larger physical memory, I don't think it > > grows large enough for, say, >256GB physical memory, to accommodate > > the necessarily large number of page tables. > > > > So, I'd vote for NOT allowing dom0 to balloon up to physical > > memory if dom0_mem is specified, and possibly a kernel command > > line option that allows it to grow beyond. Or, possibly, no > > option and never allow dom0 memory to grow beyond dom0_mem > > unless (possibly) it grows with hot-plug. > > Yes, its a bit of a problem. The trouble is that the kernel can't > really distinguish the two cases; either way, it sees a Xen-supplied > xen_start_info->nr_pages as the amount of initial memory available, and > an E820 table referring to more RAM beyond that. > > I guess there are three options: > > 1. add a "xen_maxmem" (or something) kernel parameter to override > space specified in the E820 table > 2. ignore E820 if its a privileged domain As it stands I don't think it is currently possible to boot any domain 0 kernel pre-ballooned other than by using the native mem= option. I think the Right Thing to do would be for privileged domains to combine the results of XENMEM_machine_memory_map (real e820) and XENMEM_memory_map (pseudo-physical "e820") by clamping the result of XENMEM_machine_memory_map at the maximum given in XENMEM_memory_map (or taking some sort of union). Then if we wanted to support pre-ballooning domain 0 via hypervisor only interfaces for some reason in the future then we would need to add a new option dom0_maxmem= or so which populated the result of XENMEM_memory_map with the appropriate size. I think this would be consistent with the behaviour for a non-privileged domain, dom0_mem= and dom0_maxmem= would correspond to memory= and maxmem= in a domU configuration file. However, although I think that the Right Thing, I don't think having domain 0 cut off its e820 at nr_pages unless overridden by mem= would be a problem in practice and it certainly wins in terms of complexity of reconciling XENMEM_memory_map and XENMEM_machine_memory_map. Ian.