From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:58594) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bKMwe-00057h-D5 for qemu-devel@nongnu.org; Tue, 05 Jul 2016 05:47:09 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bKMwZ-0001MB-Ux for qemu-devel@nongnu.org; Tue, 05 Jul 2016 05:47:07 -0400 Received: from mx1.redhat.com ([209.132.183.28]:59759) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bKMwZ-0001M6-Op for qemu-devel@nongnu.org; Tue, 05 Jul 2016 05:47:03 -0400 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 4098CC049D58 for ; Tue, 5 Jul 2016 09:47:03 +0000 (UTC) Date: Tue, 5 Jul 2016 10:46:59 +0100 From: "Daniel P. Berrange" Message-ID: <20160705094659.GH6553@redhat.com> Reply-To: "Daniel P. Berrange" References: <1467659769-15900-1-git-send-email-dgilbert@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1467659769-15900-1-git-send-email-dgilbert@redhat.com> Subject: Re: [Qemu-devel] [PATCH v2 0/6] x86: Physical address limit patches List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Dr. David Alan Gilbert (git)" Cc: qemu-devel@nongnu.org, pbonzini@redhat.com, ehabkost@redhat.com, marcel@redhat.com, mst@redhat.com, kraxel@redhat.com On Mon, Jul 04, 2016 at 08:16:03PM +0100, Dr. David Alan Gilbert (git) wrote: > From: "Dr. David Alan Gilbert" > > QEMU sets the guests physical address bits to 40; this is wrong > on most hardware, and can be detected by the guest. > It also stops you using really huge multi-TB VMs. > > Red Hat has had a patch, that Andrea wrote, downstream for a couple > of years that reads the hosts value and uses that in the guest. That's > correct as far as the guest sees it, and lets you create huge VMs. > > The downside, is that if you've got a mix of hosts, say an i7 and a Xeon, > life gets complicated in migration; prior to 2.6 it all apparently > worked (although a guest that looked might spot the change). > In 2.6 Paolo started checking MSR writes and they failed when the > incoming MTRR mask didn't fit. > > This series: > a) Fixes up mtrr masks so that if you're migrating between hosts > of different physical address size it tries to do something sensible. > > b) Lets you specify the guest physical address size via a CPU property, i.e. > -cpu SandyBridge,phys-bits=36 > > The default on old machine types is to use the existing 40 bits value. > > c) Lets you tell qemu to use the same setting as the host, i.e. > -cpu SandyBridge,phys-bits=0 > > This is the default on new machine types. As a general rule we've tried to say that if you pick an explicit CPU model, we're migration safe. By having the phys-bits default value always reflect the host CPU value, it feels like we've made the explicit CPU model choice less safe, just like -cpu host is. IOW, if choosing a named CPU model, it feels like we should have a corresponding fixed phys-bit value for that CPU model, even if it has to be quiet conservative (eg default to bits=36). A phys-bits=0 value should only be used with -cpu host. Regards, Daniel -- |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| |: http://libvirt.org -o- http://virt-manager.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: http://entangle-photo.org -o- http://live.gnome.org/gtk-vnc :|