From mboxrd@z Thu Jan 1 00:00:00 1970 From: Anthony Liguori Subject: Re: [Qemu-devel] Re: [PATCH 1/6] Use correct types to enable > 2G support Date: Fri, 01 Feb 2008 14:31:53 -0600 Message-ID: <47A381B9.1060707@codemonkey.ws> References: <1201818980-27534-1-git-send-email-aliguori@us.ibm.com> <1201818980-27534-2-git-send-email-aliguori@us.ibm.com> <47A2F3C7.6060409@bellard.org> <47A32E40.3000204@us.ibm.com> <47A33721.4020600@qumranet.com> <47A33BC6.3060705@us.ibm.com> <18339.15712.411486.395238@mariner.uk.xensource.com> <47A35C7E.8050309@codemonkey.ws> <20080201175750.GD22981@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: kvm-devel-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org, Ian Jackson , qemu-devel-qX2TKyscuCcdnm+yROfE0A@public.gmane.org, Avi Kivity , Paul Brook To: "Daniel P. Berrange" Return-path: In-Reply-To: <20080201175750.GD22981-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org Errors-To: kvm-devel-bounces-5NWGOfrQmneRv+LV9MX5uipxlwaOVQ5f@public.gmane.org List-Id: kvm.vger.kernel.org Daniel P. Berrange wrote: > On Fri, Feb 01, 2008 at 11:53:02AM -0600, Anthony Liguori wrote: > >> Ian Jackson wrote: >> >>> Anthony Liguori writes ("[Qemu-devel] Re: [kvm-devel] [PATCH 1/6] Use correct types to enable > 2G support"): >>> >>> >>>> The alternative is to change all the places that assume phys_ram_base + >>>> PA which I don't like very much. >>>> >>>> >>> We would ideally like to do this for Xen, at least in the places we >>> care about. (Xen uses less of the qemu tree than KVM, I think.) >>> >>> >> Support for the map cache in the Xen tree is a rather big change that >> I'm not going to attempt to support it in this patch series. >> >> I'd rather preserve the phys_ram_base + PA assumption because it allows >> us to be able to do support > 1 page DMA operations for our virtual IO >> drivers. If you break the assumption that physically contiguous memory >> in the guest is virtual contiguous memory in the host, things get pretty >> ugly. >> > > Well Xen i386 has no choice but to use the map cache, since PAE lets > i386 guests have as much as 100 GB of memory & there's no way you can > map that into QEMU's 32-bit userspace. So if virt IO has a dependancy > on contigious memory access in QEMU its not going to play nice with > Xen. > For KVM (and it sounds like QEMU), we're just making the statement that 32-bit hosts cannot support > 2GB guests. I know that's a regression for Xen but in all fairness, I did raise this as an objection when the map cache was first introduced :-) virtio could still be made to work with map cache. You would just have to change it to be able to map more than one page contiguously. As I mentioned though, it just starts getting ugly. Regards, Anthony Liguori > Dan. > ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2008. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/