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

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Message-ID: <20140706151242.GN18167@minantech.com>
Date:	Sun, 6 Jul 2014 18:12:42 +0300
From:	Gleb Natapov <gleb@...nel.org>
To:	Jan Kiszka <jan.kiszka@....de>
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 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.

> 
> 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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