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

[Paolo Bonzini]

On 10/07/20 18:02, Eduardo Habkost wrote:
> 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"?

Blocking guest_maxphyaddr < host_maxphyaddr if maxphyaddr==52 would be a
good start.  However it would block the default configuration on IceLake
processors (which is why Mohammed looked at this thing in the first place).

Paolo