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

[Paolo Bonzini]

On 09/07/20 11:55, Mohammed Gamal wrote:
>> Ideally we would simply outlaw (3), but it's hard for backward
>> compatibility reasons.  Second best solution is a flag somewhere
>> (msr, cpuid, ...) telling the guest firmware "you can use
>> GUEST_MAXPHYADDR, we guarantee it is <= HOST_MAXPHYADDR".
> Problem is GUEST_MAXPHYADDR > HOST_MAXPHYADDR is actually a supported
> configuration on some setups. Namely when memory encryption is enabled
> on AMD CPUs[1].
> 

It's not that bad since there's two MAXPHYADDRs, the one in CPUID and
the one computed internally by the kernel.  GUEST_MAXPHYADDR greater
than the host CPUID maxphyaddr is never supported.

Paolo