From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC PATCH 00/17] virtual-bus Date: Thu, 02 Apr 2009 14:48:11 +0300 Message-ID: <49D4A5FB.1020601@redhat.com> References: <20090402085253.GA29932@gondor.apana.org.au> <49D47F11.6070400@redhat.com> <49D49986.3060303@novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Herbert Xu , anthony@codemonkey.ws, andi@firstfloor.org, linux-kernel@vger.kernel.org, agraf@suse.de, pmullaney@novell.com, pmorreale@novell.com, rusty@rustcorp.com.au, netdev@vger.kernel.org, kvm@vger.kernel.org To: Gregory Haskins Return-path: Received: from mx2.redhat.com ([66.187.237.31]:44938 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756068AbZDBLs0 (ORCPT ); Thu, 2 Apr 2009 07:48:26 -0400 In-Reply-To: <49D49986.3060303@novell.com> Sender: kvm-owner@vger.kernel.org List-ID: Gregory Haskins wrote: >> There is no choice. Exiting from the guest to the kernel to userspace >> is prohibitively expensive, you can't do that on every packet. >> >> > > Now you are making my point ;) This is part of the cost of your > signaling path, and it directly adds to your latency time. It adds a microsecond. The kvm overhead of putting things in userspace is low enough, I don't know why people keep mentioning it. The problem is the kernel/user networking interfaces. > You can't > buffer packets here if the guest is only going to send one and wait for > a response and expect that to perform well. And this is precisely what > drove me to look at avoiding going back to userspace in the first place. > We're not buffering any packets. What we lack is a way to tell the guest that we're done processing all packets in the ring (IOW, re-enable notifications). -- error compiling committee.c: too many arguments to function