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    SubjectRe: [PATCH RFC 0/7] KVM: nVMX: enlightened VMCS initial implementation
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    On 18/12/2017 18:17, Vitaly Kuznetsov wrote:
    > The original author of these patches does no longer work at Red Hat, I
    > agreed to take this over and send upstream. Here is his original
    > description:
    >
    > "Makes KVM implement the enlightened VMCS feature per Hyper-V TLFS 5.0b.
    > I've measured about %5 improvement in cost of a nested VM exit (Hyper-V
    > enabled Windows Server 2016 nested in KVM)."

    Can you try reproducing this and see how much a simple CPUID loop costs in:

    * Hyper-V on Hyper-V (with enlightened VMCS, as a proxy for a full
    implementation including the clean fields mask)

    * Hyper-V on KVM, with and without enlightened VMCS

    The latest kvm/queue branch already cut a lot of the cost of a nested VM
    exit (from ~22000 to ~14000 clock cycles for KVM on KVM), so we could
    also see if Hyper-V needs shadowing of more fields.

    > This is just an initial implementation. By leveraging clean fields mask
    > we can further improve performance. I'm also interested in implementing
    > the other part of the feature: consuming enlightened VMCS when KVM is
    > running on top of Hyper-V.

    I'm also interested in consuming enlightened VMCS on Hyper-V if that can
    provide better performance for KVM on Azure.

    Paolo

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