linux-kernel - Re: [PART1 RFC v3 10/12] svm: Do not expose x2APIC when enable AVIC

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Date:	Tue, 5 Apr 2016 17:14:17 +0700
From:	Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
To:	Paolo Bonzini <pbonzini@...hat.com>,
	Radim Krčmář <rkrcmar@...hat.com>
CC:	<joro@...tes.org>, <bp@...en8.de>, <gleb@...nel.org>,
	<alex.williamson@...hat.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>, <wei@...hat.com>,
	<sherry.hurwitz@....com>
Subject: Re: [PART1 RFC v3 10/12] svm: Do not expose x2APIC when enable AVIC

Hi Paolo,

On 3/31/16 18:23, Paolo Bonzini wrote:
>
>
> 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.

Ahh, thanks.

>
>>> 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.

Let me look into this and investigate some more.

Thanks,
Suravee