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

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Date:   Sun, 18 Sep 2016 14:53:25 +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

2016-09-15 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.

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