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

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Message-ID: <CANRm+Cy_om4QHU0TyEipeZmmAGhw+uGkXQjNXXQBpUqXHtUTnA@mail.gmail.com>
Date:   Thu, 15 Sep 2016 12:25:49 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Mika Penttilä <mika.penttila@...tfour.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
        Paolo Bonzini <pbonzini@...hat.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

2016-09-15 12:08 GMT+08:00 Mika Penttilä <mika.penttila@...tfour.com>:
> On 09/14/2016 10:58 AM, 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.
>>
>
> While at it, is the vmx flexpriotity stuff still valid code?
> AFAICS it gets enabled iff TPR shadow is on. flexpriority
> is on when :
>
> (flexpriority_enabled && lapic_in_kernel && cpu_has_vmx_tpr_shadow && cpu_has_vmx_virtualize_apic_accesses)
>
> But apic accesses to TPR mmio are not then trapped and TPR changes not reported because
> the “use TPR shadow” VM-execution control is 1.

Please note the patch is for MSR-BASED TPR shadow w/o APICv, TPR
shadow can work correctly in other configurations.

Regards,
Wanpeng Li