linux-kernel - Re: [PATCH v2] enable x2APIC without interrupt remapping under KVM

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Date:	Mon, 29 Jun 2009 12:22:35 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Sheng Yang <sheng@...ux.intel.com>,
	kvm@...r.kernel.org, avi@...hat.com,
	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
Subject: Re: [PATCH v2] enable x2APIC without interrupt remapping under KVM


* Gleb Natapov <gleb@...hat.com> wrote:

> KVM would like to provide x2APIC interface to a guest without emulating
> interrupt remapping device. The reason KVM prefers guest to use x2APIC
> is that x2APIC interface is better virtualizable and provides better
> performance than mmio xAPIC interface:
> 
> - msr exits are faster than mmio (no page table walk, emulation)
> - no need to read back ICR to look at the busy bit
> - one 64 bit ICR write instead of two 32 bit writes
> - shared code with the Hyper-V paravirt interface
> 
> Included patch changes x2APIC enabling logic to enable it even if IR
> initialization failed, but kernel runs under KVM and no apic id is
> greater than 255 (if there is one spec requires BIOS to move to x2apic
> mode before starting an OS).
> 
> Signed-off-by: Gleb Natapov <gleb@...hat.com>
> ---
> 
> v2:
>   Add help message to Kconfig
>   Force physical mode if IR is not available.
> 
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index d1430ef..3e5b6ea 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -260,12 +260,15 @@ config SMP
>  
>  config X86_X2APIC
>  	bool "Support x2apic"
> -	depends on X86_LOCAL_APIC && X86_64 && INTR_REMAP
> +	depends on X86_LOCAL_APIC && X86_64

Have you tried to build this with X86_X2APIC set but INTR_REMAP 
disabled?

	Ingo
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