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

[Mika PenttilÃ]

On 09/15/2016 07:25 AM, Wanpeng Li wrote:
> 2016-09-15 12:08 GMT+08:00 Mika Penttilà <mika.penttila@xxxxxxxxxxxx>:
>> On 09/14/2016 10:58 AM, Wanpeng Li wrote:
>>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>>
>>> 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 âuse TPR shadowâ 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
> 

Hi,

Yes I see, this is slightly offtopic but while at flexpriority, how is it relevant in current kvm?
In other words I see it as dead code, because it is enabled only when TPR shadow is enabled,
and as such ineffective because TPR shadow disables the wmexits tpr reporting uses.

Thanks,
Mika