* Daniel P. Berrange (berra...@redhat.com) wrote: > On Mon, Jul 04, 2016 at 08:16:03PM +0100, Dr. David Alan Gilbert (git) wrote: > > From: "Dr. David Alan Gilbert" <dgilb...@redhat.com> > > > > 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.
phys-bits doesn't follow a cpu model in real hardware e.g. a SandyBridge Xeon and a SandyBridge i7 are different. So unless we suddenly created at least 2x as many cpu models we can't do that. Dave > > > 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 :| -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
