From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH] KVM: APIC: avoid instruction emulation for EOI writes Date: Mon, 29 Aug 2011 15:55:07 +0200 Message-ID: <4E5B9A3B.2020009@siemens.com> References: <625BA99ED14B2D499DC4E29D8138F15063045B0C0C@shsmsx502.ccr.corp.intel.com> <4E5B68DA.1090208@siemens.com> <4E5B70F8.30307@redhat.com> <4E5B7206.5070603@siemens.com> <4E5B73CC.5080800@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: "Tian, Kevin" , "kvm@vger.kernel.org" , "Nakajima, Jun" , "Dong, Eddie" , Marcelo Tosatti To: Avi Kivity Return-path: Received: from david.siemens.de ([192.35.17.14]:33723 "EHLO david.siemens.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751766Ab1H2NzR (ORCPT ); Mon, 29 Aug 2011 09:55:17 -0400 In-Reply-To: <4E5B73CC.5080800@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On 2011-08-29 13:11, Avi Kivity wrote: > On 08/29/2011 02:03 PM, Jan Kiszka wrote: >>> >>> Just reading the first byte requires a guest page table walk. This is >>> probably the highest cost in emulation (which also requires a walk for >>> the data access). >> >> And what about caching the result of the first walk? Usually, a "sane >> guest" won't have many code pages that issue the EIO. >> > > There's no way to know when to invalidate the cache. Set the affected code page read-only? > > We could go a bit further, and cache the the whole thing. On the first > exit, do the entire emulation, and remember %rip. On the second exit, > if %rip matches, skip directly to kvm_lapic_eoi(). > > But I don't think it's worth it. This also has failure modes, and > really, no guest will ever write to EOI with stosl. ...or add/sub/and/or etc. Well, we've done other crazy things in the past just to keep even the unlikely case correct. I was just wondering if that policy changed. However, I just realized that user space is able to avoid this inaccuracy for potentially insane guests by not using in-kernel irqchips. So we have at least a knob. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux