linux-kernel - Re: [PATCH] x86: Make x2APIC support depend on interrupt remapping or guest support

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Date:	Sun, 06 Jul 2014 17:24:27 +0200
From:	Jan Kiszka <jan.kiszka@....de>
To:	Gleb Natapov <gleb@...nel.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86 <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>
Subject: Re: [PATCH] x86: Make x2APIC support depend on interrupt remapping
 or guest support

On 2014-07-06 17:12, Gleb Natapov wrote:
> On Sat, Jul 05, 2014 at 09:47:54AM +0200, Jan Kiszka wrote:
>> From: Jan Kiszka <jan.kiszka@...mens.com>
>>
>> We are able to use x2APIC mode in the absence of interrupt remapping on
>> certain hypervisors. So it if fine to disable IRQ_REMAP without having
>> to give up x2APIC support.
> FWIW I did similar thing back when I added x2apic to KVM:
> http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-06/msg14579.html
> But was advised against it.

I don't get the point from that thread.

However, this change is just formalizing a configuration that is already
being used: there is no interrupt remapping available with KVM (yet),
but we do run our guests in x2APIC mode most of the time. Interrupt
remapping code in kernels tailored as KVM guest is dead code, right now.

Jan

> 
>>
>> Signed-off-by: Jan Kiszka <jan.kiszka@...mens.com>
>> ---
>>  arch/x86/Kconfig | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
>> index a8f749e..30a9987 100644
>> --- a/arch/x86/Kconfig
>> +++ b/arch/x86/Kconfig
>> @@ -310,7 +310,7 @@ config SMP
>>  
>>  config X86_X2APIC
>>  	bool "Support x2apic"
>> -	depends on X86_LOCAL_APIC && X86_64 && IRQ_REMAP
>> +	depends on X86_LOCAL_APIC && X86_64 && (IRQ_REMAP || HYPERVISOR_GUEST)
>>  	---help---
>>  	  This enables x2apic support on CPUs that have this feature.
>>  
>>
>>
> 
> 
> 
> --
> 			Gleb.
> 



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