From mboxrd@z Thu Jan 1 00:00:00 1970 From: Avi Kivity Subject: Re: [RFC PATCH 00/17] virtual-bus Date: Fri, 03 Apr 2009 14:03:45 +0300 Message-ID: <49D5ED11.2000800@redhat.com> References: <20090402085253.GA29932@gondor.apana.org.au> <49D47F11.6070400@redhat.com> <49D5EBE1.8030200@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: Herbert Xu , ghaskins@novell.com, 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: Gerd Hoffmann Return-path: Received: from mx2.redhat.com ([66.187.237.31]:33149 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763505AbZDCLDU (ORCPT ); Fri, 3 Apr 2009 07:03:20 -0400 In-Reply-To: <49D5EBE1.8030200@redhat.com> Sender: netdev-owner@vger.kernel.org List-ID: Gerd Hoffmann wrote: > Avi Kivity 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. >> > > I didn't look at virtio-net very closely yet. I wonder why the > notification is that a big issue though. It is easy to keep the number > of notifications low without increasing latency: > > Check shared ring status when stuffing a request. If there are requests > not (yet) consumed by the other end there is no need to send a > notification. That scheme can even span multiple rings (nics with rx > and tx for example). > If the host is able to consume a request immediately, and the guest is not able to batch requests, this breaks down. And that is the current situation. -- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.