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List:       kvm
Subject:    Re: [PART1 RFC v3 10/12] svm: Do not expose x2APIC when enable AVIC
From:       Paolo Bonzini <pbonzini () redhat ! com>
Date:       2016-03-31 11:23:26
Message-ID: 56FD08AE.6000201 () redhat ! com
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On 31/03/2016 06:15, Suravee Suthikulpanit wrote:
>>> +                vcpu->arch.cpuid_entries[i].ecx &= ~(1 << 21);
>> 
>> and X86_FEATURE_X2APIC (or something with X2APIC in name) for the
>> bit.
>> 
>> The code will become so obvious that the comment can be removed.
>> :)
> 
> Good point. I can only find example of using (X86_FEATURE_X2APIC %
> 32) == 21.

You can use bit(X86_FEATURE_X2APIC), it is defined in arch/x86/kvm/x86.h.

>> but the MSR interface is going to exit and host-side interrupt 
>> delivery will probably still work, so I don't see a huge problem
>> with it.
> 
> Agree that it will still work. However, in such case, the guest code
> would likely default to using x2APIC interface, which will not be
> handled by the AVIC hardware, and resulting in no performance
> improvement that we are trying to introduce.

You would still get some improvement from exit-free interrupt delivery.

Paolo
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