On Tue, Jul 05, 2016 at 10:49:48AM +0100, Dr. David Alan Gilbert wrote: > * 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.
I believe Daniel's point is not whether the CPU model matches real hardware (we don't really care), but whether its ABI is stable and migration-safe. So, replying to Daniel: the issue you point out is real, but the problem here is that both options are bad and we need to choose the least harmful one. We can't emulate properly a phys-bits=36 VM in a phys-bits=40 host, and vice versa. So keeping phys-bits the same on migration is also a bad solution. See the v1 discussion thread: Subject: Default for phys-addr-bits? (was Re: [PATCH 4/5] x86: Allow physical address bits to be set) http://article.gmane.org/gmane.comp.emulators.qemu/421132 -- Eduardo