From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Stephen Hemminger" Subject: Re: Dropping NETIF_F_SG since no checksum feature. Date: Mon, 9 Oct 2006 09:50:51 -0700 Message-ID: <20061009095051.38ed9f22@freekitty> References: <20061009174705.GG26849@mellanox.co.il> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org, "Linux Kernel Mailing List" , openib-general@openib.org Return-path: To: "Michael S. Tsirkin" In-Reply-To: <20061009174705.GG26849@mellanox.co.il> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: openib-general-bounces@openib.org Errors-To: openib-general-bounces@openib.org List-Id: netdev.vger.kernel.org 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. > 2. On the receive side, what's the best/right way to create an skb that > is larger than PAGE_SIZE? > Do I allocate with alloc_page and fill in nr_frags with skb_fill_page_desc? > Some drivers seem to fill in frag_list - which is better? > I see than even skb_put only works properly on linear skb. Allocating large buffers is problematic on busy systems. See lastest e1000 or sky2 that use frag_list. > What are the helpers legal for fragmented skb? Read the source. Setting up fragmented buffers has less helper functions, but isn't that hard. -- Stephen Hemminger