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

From: Radim KrÄmÃÅ
Date: Wed Sep 14 2016 - 08:03:35 EST

2016-09-14 11:40+0200, Paolo Bonzini:
> On 14/09/2016 09:58, Wanpeng Li wrote:
>> From: Wanpeng Li <>
>> 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).