From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: 8bf00a529967dafbbb210b377c38a15834d1e979 - performance regression? Date: Thu, 31 Oct 2013 02:21:46 +0200 Message-ID: <20131031002146.GA28569@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: gleb@redhat.com To: kvm@vger.kernel.org Return-path: Received: from mx1.redhat.com ([209.132.183.28]:56349 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751536Ab3JaASx (ORCPT ); Wed, 30 Oct 2013 20:18:53 -0400 Received: from int-mx02.intmail.prod.int.phx2.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id r9V0IrV4021381 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=OK) for ; Wed, 30 Oct 2013 20:18:53 -0400 Content-Disposition: inline Sender: kvm-owner@vger.kernel.org List-ID: 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? 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. Why that's not a large regression, it's a far cry from actually helping performance. So are we better off not using this feature? What kind of workload is improved by this change?