From mboxrd@z Thu Jan 1 00:00:00 1970 From: Gleb Natapov Subject: Re: 8bf00a529967dafbbb210b377c38a15834d1e979 - performance regression? Date: Thu, 31 Oct 2013 09:48:08 +0200 Message-ID: <20131031074808.GN4651@redhat.com> References: <20131031002146.GA28569@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: kvm@vger.kernel.org To: "Michael S. Tsirkin" Return-path: Received: from mx1.redhat.com ([209.132.183.28]:22951 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753710Ab3JaHsL (ORCPT ); Thu, 31 Oct 2013 03:48:11 -0400 Received: from int-mx10.intmail.prod.int.phx2.redhat.com (int-mx10.intmail.prod.int.phx2.redhat.com [10.5.11.23]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r9V7m9on003923 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Thu, 31 Oct 2013 03:48:10 -0400 Content-Disposition: inline In-Reply-To: <20131031002146.GA28569@redhat.com> Sender: kvm-owner@vger.kernel.org List-ID: On Thu, Oct 31, 2013 at 02:21:46AM +0200, Michael S. Tsirkin wrote: > commit 8bf00a529967dafbbb210b377c38a15834d1e979: > " KVM: VMX: add support for switching of PERF_GLOBAL_CTRL " was > as far as I can tell supposed to bring about performance improvement > on hardware that supports it? No, it (and commits after it) supposed to fix a bug which it did. > Instead it seems to make the typical case (not running guest > under perf) a bit slower than it used to be. > the cost of VMexit goes up by about 50 cycles > on sandy bridge where the optimization in question > actually is activated. > You seams to be confused. 8bf00a529967dafbbb210 adds support for special PERF_GLOBAL_CTRL switching, but does not add code to switch anything, so the commit itself is a nop. Next commit d7cd97964ba6d70c5 uses add_atomic_switch_msr()/clear_atomic_switch_msr() to switch PERF_GLOBAL_CTRL, but it does not depend on VM_(ENTRY|EXIT)_LOAD_IA32_PERF_GLOBAL_CTRL support which previous patch added, if the support is not there the switching will use another mechanism which is even slower. So MSR is switched no matter if PERF_GLOBAL_CTRL is enabled or not. If you saying that using VM_(ENTRY|EXIT)_LOAD_IA32_PERF_GLOBAL_CTRL is slower than using generic vmentry MSR switching then I pretty much doubt it since the only purpose of special VM_(ENTRY|EXIT)_LOAD_IA32_PERF_GLOBAL_CTRL is to be faster then general mechanism. -- Gleb.