From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC PATCH 0/3] generic hypercall support Date: Fri, 08 May 2009 21:50:06 +0300 Message-ID: <4A047EDE.2020806@redhat.com> References: <4A0040C0.1080102@redhat.com> <4A0041BA.6060106@novell.com> <4A004676.4050604@redhat.com> <4A0049CD.3080003@gmail.com> <20090505231718.GT3036@sequoia.sous-sol.org> <4A010927.6020207@novell.com> <20090506072212.GV3036@sequoia.sous-sol.org> <4A018DF2.6010301@novell.com> <20090506160712.GW3036@sequoia.sous-sol.org> <4A031471.7000406@novell.com> <20090507233503.GA9103@amt.cnet> <4A043E89.90403@novell.com> <4A044786.2080508@codemonkey.ws> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Gregory Haskins , Marcelo Tosatti , Chris Wright , Gregory Haskins , linux-kernel@vger.kernel.org, kvm@vger.kernel.org To: Anthony Liguori Return-path: Received: from mx2.redhat.com ([66.187.237.31]:44918 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753522AbZEHSvD (ORCPT ); Fri, 8 May 2009 14:51:03 -0400 In-Reply-To: <4A044786.2080508@codemonkey.ws> Sender: kvm-owner@vger.kernel.org List-ID: Anthony Liguori wrote: > > And we're now getting close to the point where the difference is > virtually meaningless. > > At .14us, in order to see 1% CPU overhead added from PIO vs HC, you > need 71429 exits. > If I read things correctly, you want the difference between PIO and PIOoHC, which is 210ns. But your point stands, 50,000 exits/sec will add 1% cpu overhead. > > The non-x86 architecture argument isn't valid because other > architectures either 1) don't use PCI at all (s390) and are already > using hypercalls 2) use PCI, but do not have a dedicated hypercall > instruction (PPC emb) or 3) have PIO (ia64). ia64 uses mmio to emulate pio, so the cost may be different. I agree on x86 it's almost negligible. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.