Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964826AbWJJRqo (ORCPT ); Tue, 10 Oct 2006 13:46:44 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751074AbWJJRqo (ORCPT ); Tue, 10 Oct 2006 13:46:44 -0400 Received: from smtp.osdl.org ([65.172.181.4]:1720 "EHLO smtp.osdl.org") by vger.kernel.org with ESMTP id S1751050AbWJJRqn (ORCPT ); Tue, 10 Oct 2006 13:46:43 -0400 Date: Tue, 10 Oct 2006 10:43:15 -0700 From: Stephen Hemminger To: "Michael S. Tsirkin" Cc: Linux Kernel Mailing List , netdev@vger.kernel.org, openib-general@openib.org, Roland Dreier Subject: Re: Dropping NETIF_F_SG since no checksum feature. Message-ID: <20061010104315.61540986@freekitty> In-Reply-To: <20061010144330.GA28175@mellanox.co.il> References: <20061009095051.38ed9f22@freekitty> <20061010144330.GA28175@mellanox.co.il> Organization: OSDL X-Mailer: Sylpheed-Claws 2.5.0-rc3 (GTK+ 2.10.6; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2418 Lines: 64 On Tue, 10 Oct 2006 16:43:30 +0200 "Michael S. Tsirkin" wrote: > Quoting r. Stephen Hemminger : > > Subject: Re: Dropping NETIF_F_SG since no checksum feature. > > > > On Mon, 9 Oct 2006 19:47:05 +0200 > > "Michael S. Tsirkin" wrote: > > > > > Hi! > > > I'm trying to build a network device driver supporting a very large MTU (around 64K) > > > on top of an infiniband connection, and I've hit a couple of issues I'd > > > appreciate some feedback on: > > > > > > 1. On the send side, > > > I've set NETIF_F_SG, but hardware does not support checksum offloading, > > > and I see "dropping NETIF_F_SG since no checksum feature" warning, > > > and I seem to be getting large packets all in one chunk. > > > The reason I've set NETIF_F_SG, is because I'm concerned that under real life > > > stress Linux won't be able to allocate 64K of continuous memory. > > > > > > Is this concern of mine valid? I saw in-tree drivers allocating at least 8K. > > > What's the best way to enable S/G on send side? > > > Is checksum offloading really required for S/G? > > > > Yes, in the current implementation, Linux needs checksum offload. But there > > is no reason, your driver can't compute the checksum in software. > > Are there drivers that do this already? Couldn't find any such beast ... dev_queue_xmit() does it, all you need to do in your driver is: /* If packet is not checksummed and device does not support * checksumming for this protocol, complete checksumming here. */ if (skb->ip_summed == CHECKSUM_PARTIAL) { if (skb_checksum_help(skb)) goto error_recovery } > I'm worried whether an extra pass over data won't eat up all of > the performance gains I get from the large MTU ... Yup, the cost is in touching the data, not in the copy. > > > What are the helpers legal for fragmented skb? > > BTW, I found skb_put_frags in sky2 which seems generic enough - I even wander > why isn't this in net/core. > Only because I just wrote it for my needs. If you need it, then it can be moved to skbuff.c -- Stephen Hemminger - 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/