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    SubjectRe: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv
    2016-09-14 11:40+0200, Paolo Bonzini:
    > On 14/09/2016 09:58, Wanpeng Li wrote:
    >> From: Wanpeng Li <wanpeng.li@hotmail.com>
    >>
    >> I observed that kvmvapic(to optimize flexpriority=N or AMD) is used
    >> to boost TPR access when testing kvm-unit-test/eventinj.flat tpr case
    >> on my haswell desktop (w/ flexpriority, w/o APICv). Commit (8d14695f9542
    >> x86, apicv: add virtual x2apic support) disable virtual x2apic mode
    >> completely if w/o APICv, and the author also told me that windows guest
    >> can't enter into x2apic mode when he developed the APICv feature several
    >> years ago. However, it is not truth currently, Interrupt Remapping and
    >> vIOMMU is added to qemu and the developers from Intel test windows 8 can
    >> work in x2apic mode w/ Interrupt Remapping enabled recently.
    >>
    >> This patch enables TPR shadow for virtual x2apic mode to boost
    >> windows guest in x2apic mode even if w/o APICv.
    >>
    >> Can pass the kvm-unit-test.
    >
    > Ok, now I see what you meant; this actually makes sense. I don't expect
    > much speedup though, because Linux doesn't touch the TPR and Windows is
    > likely going to use the Hyper-V APIC MSRs when APICv is disabled. For
    > this reason I'm not sure if the patch is useful in practice.

    I agree with Paolo on the use case -- what configurations benefit from
    this change?

    > To test this patch, you have to run kvm-unit-tests with Hyper-V
    > synthetic interrupt enabled. Did you do this?

    The patch is buggy. MSR bitmaps are global and we'd have a CVE if one
    guests used synic (=> disabled apicv) and one didn't.
    You'd want a new set of bitmaps and assign them in vmx_set_msr_bitmap()
    (or completely rewrite our management).

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