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Subject: Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR < host MAXPHYADDR
To:     Jim Mattson <jmattson@google.com>,
        Mohammed Gamal <mgamal@redhat.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>
References: <20200710154811.418214-1-mgamal@redhat.com>
 <CALMp9eRfZ50iyrED0-LU75VWhHu_kVoB2Qw55VzEFzZ=0QCGow@mail.gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <0c892b1e-6fe6-2aa7-602e-f5fadc54c257@redhat.com>
Date:   Fri, 10 Jul 2020 19:06:05 +0200
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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).

Paolo