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From:   Jim Mattson <jmattson@google.com>
Date:   Fri, 10 Jul 2020 10:26:40 -0700
Message-ID: <CALMp9eRGTedw-wNL4HcrJOpAQBa_Qsqcir0BtJ-dEO6EhvWGqA@mail.gmail.com>
Subject: Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR < host MAXPHYADDR
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Mohammed Gamal <mgamal@redhat.com>, 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>
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On Fri, Jul 10, 2020 at 10:16 AM Paolo Bonzini <pbonzini@redhat.com> wrote:
>
> 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).

We all know that the direct map is evil. :-)

Sorry it took me so long to get there. I didn't realize that Linux was
incapable of using more physical memory than it could map into the
kernel's virtual address space. (Wasn't that the whole point of PAE
originally?)