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

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Date:   Fri, 10 Jul 2020 10:26:28 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Mohammed Gamal <mgamal@...hat.com>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3 0/9] KVM: Support guest MAXPHYADDR < host MAXPHYADDR

On Fri, Jul 10, 2020 at 07:16:14PM +0200, Paolo Bonzini wrote:
> 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.

Yeah, the whole 5-level vs. 4-level thing needs clarification.  Using 5-level
doesn't magically change the host's MAXPA.  But using 5-level vs. 4-level EPT
does change the guest's effective MAXPA.

If the changelog is referring purely to host MAXPA, then just explicitly
state that and don't even mention 5-level vs. 4-level.