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From: Wanpeng Li <kernellwp@gmail.com>
To: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	kvm <kvm@vger.kernel.org>, Wanpeng Li <wanpeng.li@hotmail.com>,
	Wincy Van <fanwenyi0529@gmail.com>,
	Yang Zhang <yang.zhang.wz@gmail.com>
Subject: Re: [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv
Date: Thu, 15 Sep 2016 09:19:13 +0800	[thread overview]
Message-ID: <CANRm+CxO1UXW9BZdt7mDjtyAhCOKKg4WO5w3gp1RhSE=pMOHHw@mail.gmail.com> (raw)
In-Reply-To: <20160914120325.GG15680@potion>

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?

Old windows guest w/o Hyper-V synthetic interrupt support.

>
>> 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).

  reply	other threads:[~2016-09-15  1:19 UTC|newest]

Thread overview: 19+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-09-14  7:58 [PATCH] KVM: VMX: Enable MSR-BASED TPR shadow even if w/o APICv Wanpeng Li
2016-09-14  9:40 ` Paolo Bonzini
2016-09-14 12:03   ` Radim Krčmář
2016-09-15  1:19     ` Wanpeng Li [this message]
2016-09-15  6:29       ` Paolo Bonzini
2016-09-15  6:40         ` Wanpeng Li
2016-09-15  7:05     ` Wanpeng Li
2016-09-15 15:58       ` Radim Krčmář
2016-09-18  6:53         ` Wanpeng Li
2016-09-18  6:55           ` Wanpeng Li
2016-09-19 13:44           ` Radim Krčmář
2016-09-20  0:18             ` Wanpeng Li
2016-09-22  6:48             ` Wanpeng Li
2016-09-15  0:07   ` Wanpeng Li
2016-09-15  0:17     ` Wanpeng Li
2016-09-15  4:08 ` Mika Penttilä
2016-09-15  4:25   ` Wanpeng Li
2016-09-15  4:46     ` Mika Penttilä
2016-09-15  6:30   ` Paolo Bonzini

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