Re: [Qemu-devel] [PATCH 4/5] x86: Allow physical address bits to be set

[Eduardo Habkost]

On Fri, Jun 17, 2016 at 05:28:03PM +0200, Paolo Bonzini wrote:
> On 17/06/2016 17:19, Eduardo Habkost wrote:
> > > > So, what happens if the guest play tricks in bits 40-45 when QEMU
> > > > sets the limit to 40 but we are running in a 46-bit host? Is it
> > > > really a problem? I assumed it would be safe.
> > > 
> > > The guest expects a "reserved bit set" page fault, but doesn't get one.
> > 
> > Wait, are you talking about migration only, or are you really
> > talking about running current QEMU (hardcoded to 40) on a 46-bit
> > host? I'm not talking about migration, above.
> 
> I'm talking about both. :(
> 
> > We really can't emulate a 40-bit machine in a 46-bit host? I
> > didn't expect that.
> 
> Unfortunately that's the case. :(

OK, so please ignore all my suggestions about choosing reasonable
defaults based on VM size, etc.

Now all options look bad, and we need to choose the least harmful
as the default... I need to re-read this thread to be able to
give an opinion.

-- 
Eduardo