Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756929Ab1ERKiW (ORCPT ); Wed, 18 May 2011 06:38:22 -0400 Received: from mx1.redhat.com ([209.132.183.28]:3993 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756755Ab1ERKiU (ORCPT ); Wed, 18 May 2011 06:38:20 -0400 Date: Wed, 18 May 2011 13:38:19 +0300 From: "Michael S. Tsirkin" To: Shirley Ma Cc: =?utf-8?B?TWljaGHFgiBNaXJvc8WCYXc=?= , Ben Hutchings , David Miller , Eric Dumazet , Avi Kivity , Arnd Bergmann , netdev@vger.kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH V5 2/6 net-next] netdevice.h: Add zero-copy flag in netdevice Message-ID: <20110518103819.GL7589@redhat.com> References: <1305574128.3456.23.camel@localhost.localdomain> <1305574518.2885.25.camel@bwh-desktop> <1305574680.3456.33.camel@localhost.localdomain> <1305575253.2885.28.camel@bwh-desktop> <20110516211459.GE18148@redhat.com> <1305588738.3456.65.camel@localhost.localdomain> <1305671318.10756.49.camel@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1305671318.10756.49.camel@localhost.localdomain> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1629 Lines: 44 On Tue, May 17, 2011 at 03:28:38PM -0700, Shirley Ma wrote: > On Tue, 2011-05-17 at 23:48 +0200, Michał Mirosław wrote: > > 2011/5/17 Shirley Ma : > > > Hello Michael, > > > > > > Looks like to use a new flag requires more time/work. I am thinking > > > whether we can just use HIGHDMA flag to enable zero-copy in macvtap > > to > > > avoid the new flag for now since mavctap uses real NICs as lower > > device? > > > > Is there any other restriction besides requiring driver to not recycle > > the skb? Are there any drivers that recycle TX skbs? Not just recycling skbs, keeping reference to any of the pages in the skb. Another requirement is to invoke the callback in a timely fashion. For example virtio-net doesn't limit the time until that happens (skbs are only freed when some other packet is transmitted), so we need to avoid zcopy for such (nested-virt) scenarious, right? > > Not more other restrictions, skb clone is OK. pskb_expand_head() looks > OK to me from code review. Hmm. pskb_expand_head calls skb_release_data while keeping references to pages. How is that ok? What do I miss? > Currently there is no drivers recycle TX skbs. > > Thanks > Shirley Well, with e.g. bridge or veth the skb might enter the host networking stack. Once that happens, we lose track of the pages. Or is there anything that prevents such setups? -- MST -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/