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SubjectRe: [PATCH RFC 2/7] KVM: nVMX: modify vmcs12 fields to match Hyper-V enlightened VMCS
You can change the default VMCS12_REVISION and associated layout, as
long as support is maintained for the old layout and userspace has the
ability (e.g. by setting the IA32_VMX_BASIC MSR) to specify that a VM
needs to use the old layout.

On Tue, Dec 19, 2017 at 4:25 AM, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> Jim Mattson <jmattson@google.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

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