Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753330Ab0H0AGB (ORCPT ); Thu, 26 Aug 2010 20:06:01 -0400 Received: from 74-93-104-97-Washington.hfc.comcastbusiness.net ([74.93.104.97]:53347 "EHLO sunset.davemloft.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752015Ab0H0AF7 (ORCPT ); Thu, 26 Aug 2010 20:05:59 -0400 Date: Thu, 26 Aug 2010 17:06:14 -0700 (PDT) Message-Id: <20100826.170614.02282407.davem@davemloft.net> To: shemminger@vyatta.com Cc: tsi@ualberta.ca, eric.dumazet@gmail.com, bhutchings@solarflare.com, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, kuznet@ms2.inr.ac.ru, pekkas@netcore.fi, jmorris@namei.org, yoshfuji@linux-ipv6.org, kaber@trash.net Subject: Re: RFC: MTU for serving NFS on Infiniband From: David Miller In-Reply-To: <20100826165359.3b79b27d@nehalam> References: <1282823827.2476.663.camel@edumazet-laptop> <20100826165359.3b79b27d@nehalam> X-Mailer: Mew version 6.3 on Emacs 23.1 / Mule 6.0 (HANACHIRUSATO) Mime-Version: 1.0 Content-Type: Text/Plain; charset=us-ascii Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1226 Lines: 29 From: Stephen Hemminger Date: Thu, 26 Aug 2010 16:53:59 -0700 > On Thu, 26 Aug 2010 08:43:42 -0600 (Mountain Daylight Time) > Marc Aurele La France wrote: > >> Right. And a 65520 MTU allocates sk_buff's with 128K contiguous payloads. > > Infiniband device driver needs to be fixed to do SG and checksum offload. Agreed, this problem is in the infiniband layer and should be fixed there. But I fear there is a real potential blocker for this, if the infiniband layer can't checksum transmit packets in hardware we cannot legitimately add SG support. Paged SKBs can have references to page cache pages and similar. These can be updated asynchronously to the transmit, there is no locking at all to freeze the contents, and therefore full checksum offload is required to support SG correctly. So don't get the idea to do the checksum in software in the infiniband layer, and advertize hw checksumming support, to get around this :-) -- 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/