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

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Message-ID: <CANRm+Cwn80pqpk-ctkgLeRQcFZ6b=_c7jcC7DNSzCao_hNen+w@mail.gmail.com>
Date:   Sun, 18 Sep 2016 14:55:37 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...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

2016-09-18 14:53 GMT+08:00 Wanpeng Li <kernellwp@...il.com>:
> 2016-09-15 23:58 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> 2016-09-15 15:05+0800, Wanpeng Li:
>>> 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?
>>>>
>>>>> 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).
>>>
>>> Do you think introduce per-VM x2apic msr bitmap make sense?
>>
>> Not much.  It would still need different msr bitmaps for VCPUs in
>> various modes, so it would take more memory and be slower without giving
>> nicer code as we'd have to do pretty much the same as we do now.
>> We could improve clarity of the caching solution instead ...
>>
>> Per-VCPU could allow a slightly clearer design, but that is very
>> wasteful -- the caching isn't that bad.
>
> Could you elaborate the caching design in your mind? :) In addition,
> I'm not sure whether we still can get benefit from x2apic tpr shadow
> w/ APICv since the overhead of the other bitmaps/caching.

w/o

>
> Btw, I heard from Tianyu from Intel, you said there was a x2apic bug
> in kvm forum and the bug maybe in kvm, I guess I meet the same bug
> when run a windows guest(server version of windows 7, 2008 or 2012) w/
> x2apic enabled in guest and -machine q35,kernel_irqchip=spit -device
> intel-iommu, intremap=on in the QEMU command line.
>
> Regards,
> Wanpeng Li