From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751515Ab1GYHnN (ORCPT ); Mon, 25 Jul 2011 03:43:13 -0400 Received: from mx1.redhat.com ([209.132.183.28]:48378 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751392Ab1GYHnG (ORCPT ); Mon, 25 Jul 2011 03:43:06 -0400 Message-ID: <4E2D1E57.1080404@redhat.com> Date: Mon, 25 Jul 2011 10:42:15 +0300 From: Avi Kivity User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.18) Gecko/20110621 Fedora/3.1.11-1.fc15 Thunderbird/3.1.11 MIME-Version: 1.0 To: Ingo Molnar CC: Andrew Lutomirski , linux-kernel@vger.kernel.org, x86 , Linus Torvalds , Arjan van de Ven Subject: Re: [RFC] syscall calling convention, stts/clts, and xstate latency References: <20110724211526.GA6785@elte.hu> In-Reply-To: <20110724211526.GA6785@elte.hu> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 07/25/2011 12:15 AM, Ingo Molnar wrote: > > All of this makes me think that, at least on Sandy Bridge, lazy > > xstate saving is a bad optimization -- if the cache is being nice, > > save/restore is faster than twiddling the TS bit. And the cost of > > the trap when TS is set blows everything else away. > > Interesting. Mind cooking up a delazying patch and measure it on > native as well? KVM generally makes exceptions more expensive, so the > effect of lazy exceptions might be less on native. While this is true in general, kvm will trap #NM only after a host context switch or an exit to host userspace. These are supposedly rare so you won't see them a lot, especially in a benchmark scenario with just one guest. ("host context switch" includes switching to the idle thread when the guest executes HLT, something I tried to optimize in the past but it proved too difficult for the gain) -- error compiling committee.c: too many arguments to function