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Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv

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2016-09-14 20:03 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>:
> 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?

Old windows guest w/o Hyper-V synthetic interrupt support.

>
>> 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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[PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-14 10:00 +0200
  Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Paolo Bonzini <pbonzini@redhat.com> - 2016-09-14 11:50 +0200
    Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Radim Krčmář <rkrcmar@redhat.com> - 2016-09-14 14:10 +0200
      Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 03:20 +0200
        Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Paolo Bonzini <pbonzini@redhat.com> - 2016-09-15 08:30 +0200
          Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 08:50 +0200
      Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 09:10 +0200
        Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Radim Krčmář <rkrcmar@redhat.com> - 2016-09-15 18:00 +0200
          Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-18 09:00 +0200
            Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-18 09:00 +0200
            Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Radim Krčmář <rkrcmar@redhat.com> - 2016-09-19 15:50 +0200
              Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-20 02:20 +0200
              Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-22 08:50 +0200
    Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 02:10 +0200
      Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 02:20 +0200
  Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 06:30 +0200
  Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Paolo Bonzini <pbonzini@redhat.com> - 2016-09-15 08:40 +0200

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