linux-kernel - Re: [PATCH] KVM: SVM: compile out AVIC if !CONFIG_X86_LOCAL

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Date:	Tue, 14 Jun 2016 17:22:24 -0400 (EDT)
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Suravee Suthikulanit <suravee.suthikulpanit@....com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	rkrcmar@...hat.com
Subject: Re: [PATCH] KVM: SVM: compile out AVIC if !CONFIG_X86_LOCAL_APIC



----- Original Message -----
> From: "Suravee Suthikulanit" <suravee.suthikulpanit@....com>
> To: "Paolo Bonzini" <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
> Cc: rkrcmar@...hat.com
> Sent: Tuesday, June 14, 2016 8:20:30 PM
> Subject: Re: [PATCH] KVM: SVM: compile out AVIC if !CONFIG_X86_LOCAL_APIC
> 
> Hi Paolo,
> 
> On 6/14/2016 11:40 AM, Paolo Bonzini wrote:
> > AVIC needs __default_cpu_present_to_apicid.  Stub out all functions
> > that use it, and disable the module parameter, if Linux is
> > compiled without local APIC support.
> 
> I think you are right that we should disable AVIC #ifndef
> CONFIG_X86_LOCAL_APIC. However, do you think we should just use
> default_cpu_present_to_apicid() instead of the
> __default_cpu_present_to_apicid()?

I'm not sure why that would help?  default_cpu_present_to_apicid
is also declared only if CONFIG_X86_LOCAL_APIC is defined.

> As for disabling AVIC, I think we can also do:
> 
>      if (!IS_ENABLED(CONFIG_X86_LOCAL_APIC))
>          avic = false;

Yes, we'll need to do that once AVIC is enabled by default; or

static bool avic = IS_ENABLED(CONFIG_X86_LOCAL_APIC);

In any case the module parameter must be hidden if there's no
support in the kernel for the local APIC.

Paolo