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

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Message-ID: <264e471d-5ca7-f7fb-b34c-3cc9c68a29da@redhat.com>
Date:   Thu, 15 Sep 2016 08:29:38 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <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



On 15/09/2016 03:19, Wanpeng Li wrote:
> 2016-09-14 20:03 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> 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?
> 
> Old windows guest w/o Hyper-V synthetic interrupt support.

... but with Hyper-V synthetic interrupt support enabled in the QEMU
command line.  Right?

Paolo

>>
>>> 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).
>