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

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Message-ID: <CANRm+Cz8ZExsjoF9pJhs4-MXkc0pN976DcbFHZ=fWf2dRNy1Ew@mail.gmail.com>
Date:   Thu, 15 Sep 2016 14:40:10 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        "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

2016-09-15 14:29 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> 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?

I think so. :)

Regards,
Wanpeng Li