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

From	Wanpeng Li <kernellwp@gmail.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv
Date	2016-09-18 09:00 +0200
Message-ID	<siEeZ-4JW-15@gated-at.bofh.it> (permalink)
References	(1 earlier) <sheZk-7DT-5@gated-at.bofh.it> <shhaO-GA-25@gated-at.bofh.it> <shyY1-3SK-15@gated-at.bofh.it> <shHeW-rB-43@gated-at.bofh.it> <siEeZ-4JW-3@gated-at.bofh.it>
Organization	linux.* mail to news gateway

Show all headers | View raw

2016-09-18 14:53 GMT+08:00 Wanpeng Li <kernellwp@gmail.com>:
> 2016-09-15 23:58 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>:
>> 2016-09-15 15:05+0800, Wanpeng Li:
>>> 2016-09-14 20:03 GMT+08:00 Radim Krčmář <rkrcmar@redhat.com>:
>>>> 2016-09-14 11:40+0200, Paolo Bonzini:
>>>>> On 14/09/2016 09:58, Wanpeng Li wrote:
>>>>>> From: Wanpeng Li <wanpeng.li@hotmail.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

Thread

[PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-14 10:00 +0200
  Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Paolo Bonzini <pbonzini@redhat.com> - 2016-09-14 11:50 +0200
    Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Radim Krčmář <rkrcmar@redhat.com> - 2016-09-14 14:10 +0200
      Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 03:20 +0200
        Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Paolo Bonzini <pbonzini@redhat.com> - 2016-09-15 08:30 +0200
          Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 08:50 +0200
      Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 09:10 +0200
        Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Radim Krčmář <rkrcmar@redhat.com> - 2016-09-15 18:00 +0200
          Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-18 09:00 +0200
            Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-18 09:00 +0200
            Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Radim Krčmář <rkrcmar@redhat.com> - 2016-09-19 15:50 +0200
              Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-20 02:20 +0200
              Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-22 08:50 +0200
    Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 02:10 +0200
      Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 02:20 +0200
  Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li <kernellwp@gmail.com> - 2016-09-15 06:30 +0200
  Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Paolo Bonzini <pbonzini@redhat.com> - 2016-09-15 08:40 +0200

csiph-web