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 08:07:52 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
        Radim Krčmář <rkrcmar@...hat.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

2016-09-14 17:40 GMT+08:00 Paolo Bonzini <pbonzini@...hat.com>:
>
>
> 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.

We should use more newer windows guests which have Hyper-V synthetic
interrupt support, however, older windows guests can't get benefit.

>
> To test this patch, you have to run kvm-unit-tests with Hyper-V
> synthetic interrupt enabled.  Did you do this?

qemu-system-x86_64 -enable-kvm -machine kernel_irqchip=split -cpu
kvm64,hv_synic -device pc-testdev -device
isa-debug-exit,iobase=0xf4,iosize=0x4 -vnc none -serial stdio -device
pci-testdev -kernel x86/hyperv_synic.flat
enabling apic
paging enabled
cr0 = 80010011
cr3 = 7fff000
cr4 = 20
enabling apic
ncpus = 1
prepare
test 0 -> 0

I run ./x86-run x86/hyperv_synic.flat against latest linus tree, it
just stuck here.

Regards,
Wanpeng Li