From mboxrd@z Thu Jan 1 00:00:00 1970 From: ebiederm@xmission.com (Eric W. Biederman) Subject: Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM Date: Tue, 30 Jun 2009 02:24:05 -0700 Message-ID: References: <20090629132926.GB20289@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: "linux-kernel\@vger.kernel.org" , Suresh Siddha , Sheng Yang , "kvm\@vger.kernel.org" , "avi\@redhat.com" To: Gleb Natapov Return-path: Received: from out01.mta.xmission.com ([166.70.13.231]:56931 "EHLO out01.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752674AbZF3JYG (ORCPT ); Tue, 30 Jun 2009 05:24:06 -0400 In-Reply-To: <20090629132926.GB20289@redhat.com> (Gleb Natapov's message of "Mon\, 29 Jun 2009 16\:29\:26 +0300") Sender: kvm-owner@vger.kernel.org List-ID: Gleb Natapov writes: > 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). How common is hotplug hardware in kvm? In particular hotplug cpus? To support that seriously you need interrupt remapping. Eric