linux-kernel - Re: [PATCH RFC 2/7] KVM: nVMX: modify vmcs12 fields to match Hyper-V enlightened VMCS

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Date:   Tue, 19 Dec 2017 13:25:11 +0100
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        "the arch\/x86 maintainers" <x86@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "Michael Kelley \(EOSG\)" <Michael.H.Kelley@...rosoft.com>,
        Mohammed Gamal <mmorsy@...hat.com>,
        Cathy Avery <cavery@...hat.com>, Bandan Das <bsd@...hat.com>,
        Roman Kagan <rkagan@...tuozzo.com>,
        LKML <linux-kernel@...r.kernel.org>, devel@...uxdriverproject.org
Subject: Re: [PATCH RFC 2/7] KVM: nVMX: modify vmcs12 fields to match Hyper-V enlightened VMCS

Jim Mattson <jmattson@...gle.com> writes:

> At this point in time, I don't think you can just blithely change the
> virtual VMCS layout and revision number. Existing VMs using the old
> layout and revision number must continue to work on versions of kvm
> past this point. You could tie the layout and revision number changes
> to KVM_CAP_HYPERV_ENLIGHTENED_VMCS if you like, but kvm must be able
> to continue to service VMs using the previous layout and revision
> number in perpetuity.
>

I see what you mean. In case we need to keep migration of nested
workloads working between KVMs of different versions we can't (ever)
touch vmcs12.

The way to go in this case, I think, is to create a completely separate
enlightened_vmcs12 struct and use it when appropriate. We can't possibly
support migrating workloads which use enlightened VMCS to an old KVM
which doesn't support it.

P.S. "If there are changes in this struct, VMCS12_REVISION must be
changed." comment needs to be replaced with "Don't even think about
changing this" :-)

-- 
  Vitaly