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    SubjectRe: [RFC 0/33] KVM: x86: hyperv: Introduce VSM support
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    On Wed Nov 8, 2023 at 11:40 AM UTC, Alexander Graf wrote:
    > Hey Nicolas,

    [...]

    > > The series is accompanied by two repositories:
    > > - A PoC QEMU implementation of VSM [3].
    > > - VSM kvm-unit-tests [4].
    > >
    > > Note that this isn't a full VSM implementation. For now it only supports
    > > 2 VTLs, and only runs on uniprocessor guests. It is capable of booting
    > > Windows Sever 2016/2019, but is unstable during runtime.
    >
    > How much of these limitations are inherent in the current set of
    > patches? What is missing to go beyond 2 VTLs and into SMP land? Anything
    > that will require API changes?

    The main KVM concepts introduced by this series are ready to deal with
    any number of VTLs (APIC ID groups, VTL KVM device).

    KVM_HV_GET_VSM_STATE should provide a copy of 'vsm_code_page_offsets'
    per-VTL, since the hypercall page is partition wide but per-VTL.
    Attaching that information as a VTL KVM device attribute fits that
    requirement nicely. I'd prefer going that way especially if the VTL KVM
    device has a decent reception. Also, the secure memory intecepts and
    HVCALL_TRANSLATE_VIRTUAL_ADDRESS take some VTL related shortcuts, but
    those are going away. Otherwise, I don't see any necessary in-kernel
    changes.

    When virtualizing Windows with VSM I've never seen usages that go beyond
    VTL1. So enabling VTL > 1 will be mostly a kvm-unit-tests effort. As for
    SMP, it just a matter of work. Notably HvStartVirtualProcessor and
    HvGetVpIndexFromApicId need to be implemented, and making sure the QEMU
    VTL scheduling code holds up.

    Nicolas

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