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

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Date:   Wed, 14 Sep 2016 14:03:25 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Wanpeng Li <wanpeng.li@...mail.com>,
        Wincy Van <fanwenyi0529@...il.com>,
        Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [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@...mail.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).