Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR < host MAXPHYADDR

Paolo Bonzini
2020-07-10T17:16:23Z

On 10/07/20 19:13, Jim Mattson wrote:
> On Fri, Jul 10, 2020 at 10:06 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>>
>> On 10/07/20 18:30, Jim Mattson wrote:
>>>>
>>>> This can be problem when having a mixed setup of machines with 5-level page
>>>> tables and machines with 4-level page tables, as live migration can change
>>>> MAXPHYADDR while the guest runs, which can theoretically introduce bugs.
>>>
>>> Huh? Changing MAXPHYADDR while the guest runs should be illegal. Or
>>> have I missed some peculiarity of LA57 that makes MAXPHYADDR a dynamic
>>> CPUID information field?
>>
>> Changing _host_ MAXPHYADDR while the guest runs, such as if you migrate
>> from a host-maxphyaddr==46 to a host-maxphyaddr==52 machine (while
>> keeping guest-maxphyaddr==46).
>
> Ah, but what does that have to do with LA57?

Intel only has MAXPHYADDR > 46 on LA57 machines (because in general OSes
like to have a physical 1:1 map into the kernel part of the virtual
address space, so having a higher MAXPHYADDR would be of limited use
with 48-bit linear addresses).

In other words, while this issue has existed forever it could be ignored
until IceLake introduced MAXPHYADDR==52 machines. I'll introduce
something like this in a commit message.

Paolo