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

From: Radim KrÄmÃÅ
Date: Mon Sep 19 2016 - 09:44:57 EST

2016-09-18 14:53+0800, Wanpeng Li:
> 2016-09-15 23:58 GMT+08:00 Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>:
>> 2016-09-15 15:05+0800, Wanpeng Li:
>>> 2016-09-14 20:03 GMT+08:00 Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>:
>>>> 2016-09-14 11:40+0200, Paolo Bonzini:
>>>>> On 14/09/2016 09:58, Wanpeng Li wrote:
>>>>>> From: Wanpeng Li <>
>>>>>> 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? :)

The one we already do -- precompute all possible bitmaps at KVM
initialization and assign the appropriate ones at runtime.

> In addition,
> I'm not sure whether we still can get benefit from x2apic tpr shadow
> w/o APICv since the overhead of the other bitmaps/caching.

Overhead from assigning a cached MSR bitmap should be less than one VM
exit caused by a TPR write when there are no pending interrupts.
Are there other sources of overhead?

> 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.

Does it happen when you are running less than 8 VCPUs (max APIC ID < 8)?
QEMU always enabled x2APIC support in IOMMU (EIME) even though it
doesn't work under some configurations.