linux-kernel - Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR

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Date:   Fri, 10 Jul 2020 19:16:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Mohammed Gamal <mgamal@...hat.com>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR < host MAXPHYADDR

On 10/07/20 19:13, Jim Mattson wrote:
> On Fri, Jul 10, 2020 at 10:06 AM Paolo Bonzini <pbonzini@...hat.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