From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [PATCH v2] enable x2APIC without interrupt remapping under KVM Date: Mon, 29 Jun 2009 12:22:35 +0200 Message-ID: <20090629102235.GA15372@elte.hu> References: <20090629100837.GW20289@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, Sheng Yang , kvm@vger.kernel.org, avi@redhat.com, "Siddha, Suresh B" To: Gleb Natapov Return-path: Received: from mx2.mail.elte.hu ([157.181.151.9]:33182 "EHLO mx2.mail.elte.hu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758856AbZF2KWk (ORCPT ); Mon, 29 Jun 2009 06:22:40 -0400 Content-Disposition: inline In-Reply-To: <20090629100837.GW20289@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: * Gleb Natapov 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 > --- > > 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