From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753120Ab0IOKQJ (ORCPT ); Wed, 15 Sep 2010 06:16:09 -0400 Received: from mx1.redhat.com ([209.132.183.28]:36954 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751523Ab0IOKQI (ORCPT ); Wed, 15 Sep 2010 06:16:08 -0400 Date: Wed, 15 Sep 2010 12:10:00 +0200 From: "Michael S. Tsirkin" To: Shirley Ma Cc: Arnd Bergmann , Avi Kivity , "Xin, Xiaohui" , David Miller , netdev@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH 2/2] macvtap: TX zero copy between guest and host kernel Message-ID: <20100915101000.GB28016@redhat.com> References: <20100914152231.GA13105@redhat.com> <1284480025.13351.49.camel@localhost.localdomain> <20100914162952.GB13560@redhat.com> <1284483745.13351.71.camel@localhost.localdomain> <20100914182707.GB15549@redhat.com> <1284490143.13351.82.camel@localhost.localdomain> <20100914190156.GA16037@redhat.com> <1284492983.13351.95.camel@localhost.localdomain> <20100915051241.GA25340@redhat.com> <1284531675.24603.259.camel@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1284531675.24603.259.camel@localhost.localdomain> User-Agent: Mutt/1.5.20 (2009-12-10) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Sep 14, 2010 at 11:21:15PM -0700, Shirley Ma wrote: > On Wed, 2010-09-15 at 07:12 +0200, Michael S. Tsirkin wrote: > > Yes, I agree this patch is useful for demo purposes: > > simple, and shows what kind of performance gains > > we can expect for TX. > > Any other issue you can see in this patch beside vhost descriptors > update? Another issue is that macvtap can be bound to almost anything, including e.g. a tap device or a bridge, which might hang on to skb fragments for unlimited time. Zero copy TX won't easily work there. I can imagine either somehow triggering a data copy after the fact (hard), or detecting such devices and avoiding zero copy (unfortunate for guest to guest, and drivers will need tuning). > Don't you think once I address vhost_add_used_and_signal update > issue, it is a simple and complete patch for macvtap TX zero copy? > > Thanks > Shirley I like the fact that the patch is simple. Unfortunately I suspect it'll stop being simple by the time it's complete :) -- MST