From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753140AbZF3TDT (ORCPT ); Tue, 30 Jun 2009 15:03:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752867AbZF3TDI (ORCPT ); Tue, 30 Jun 2009 15:03:08 -0400 Received: from mx2.redhat.com ([66.187.237.31]:55008 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752596AbZF3TDH (ORCPT ); Tue, 30 Jun 2009 15:03:07 -0400 Date: Tue, 30 Jun 2009 22:03:07 +0300 From: Gleb Natapov To: "Eric W. Biederman" Cc: "linux-kernel@vger.kernel.org" , Suresh Siddha , Sheng Yang , "kvm@vger.kernel.org" , "avi@redhat.com" Subject: Re: [PATCH v3] enable x2APIC without interrupt remapping under KVM Message-ID: <20090630190307.GE8122@redhat.com> References: <20090629132926.GB20289@redhat.com> <20090630092623.GI20289@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 30, 2009 at 09:44:54AM -0700, Eric W. Biederman wrote: > Gleb Natapov writes: > > > On Tue, Jun 30, 2009 at 02:24:05AM -0700, Eric W. Biederman wrote: > >> 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? > >> > > It works for Linux guests. > > > > >> To support that seriously you need interrupt remapping. > >> > > Can you explain why? > > Because ioapics don't fully function according to spec, > and the interrupt code on the hotplug path is a horrible > terrible broken hack for ioapics. > Considering that interrupt remapping is fairly new feature are you saying that hotplug (pci and cpu) on x86 is a horrible hack on Linux? > It is better than nothing but it certainly is not something > I would expect to work all of the time. > Because of horrible code or non-complaint ioapic implementation out there? If later this is not a big issue for KVM. > Interrupt remapping is the one case where we have hardware > that works according to spec and that works reasonably well. > I am sure when there was only one ioapic implementation in existence it worked according to a spec (and if not spec was changed :)) Give interrupt remapping some time and than we will see how above statement holds. -- Gleb.