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From:   Jim Mattson <jmattson@google.com>
Date:   Fri, 10 Jul 2020 09:30:55 -0700
Message-ID: <CALMp9eRfZ50iyrED0-LU75VWhHu_kVoB2Qw55VzEFzZ=0QCGow@mail.gmail.com>
Subject: Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR < host MAXPHYADDR
To:     Mohammed Gamal <mgamal@redhat.com>
Cc:     kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        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>
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On Fri, Jul 10, 2020 at 8:48 AM Mohammed Gamal <mgamal@redhat.com> wrote:
>
> When EPT is enabled, KVM does not really look at guest physical
> address size. Address bits above maximum physical memory size are reserved.
> Because KVM does not look at these guest physical addresses, it currently
> effectively supports guest physical address sizes equal to the host.
>
> 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?

> In this patch series we add checks on guest physical addresses in EPT
> violation/misconfig and NPF vmexits and if needed inject the proper
> page faults in the guest.
>
> A more subtle issue is when the host MAXPHYADDR is larger than that of the
> guest. Page faults caused by reserved bits on the guest won't cause an EPT
> violation/NPF and hence we also check guest MAXPHYADDR and add PFERR_RSVD_MASK
> error code to the page fault if needed.