Re: [PATCH 2/2] x86/cpu: Handle GUEST_MAXPHYADDR < HOST_MAXPHYADDR for hosts that don't support it

On Fri, Jul 10, 2020 at 09:22:42AM +0200, Paolo Bonzini wrote: > On 09/07/20 21:13, Eduardo Habkost wrote: >>> Doesn't this require intercepting MOV-to-CR3 when the guest is in PAE >>> mode, so that the hypervisor can validate the high bits in the PDPTEs? >> If the fix has additional overhead, is the additional overhead >> bad enough to warrant making it optional? Most existing >> GUEST_MAXPHYADDR < HOST_MAXPHYADDR guests already work today >> without the fix. > > The problematic case is when host maxphyaddr is 52. That case wouldn't > work at all without the fix. What can QEMU do to do differentiate "can't work at all without the fix" from "not the best idea, but will probably work"?