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

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Date:   Thu, 15 Sep 2016 17:58:58 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.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

2016-09-15 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.