Messages in this thread | | | Date | Tue, 10 Oct 2006 10:43:15 -0700 | From | Stephen Hemminger <> | Subject | Re: Dropping NETIF_F_SG since no checksum feature. |
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On Tue, 10 Oct 2006 16:43:30 +0200 "Michael S. Tsirkin" <mst@mellanox.co.il> wrote:
> Quoting r. Stephen Hemminger <shemminger@osdl.org>: > > Subject: Re: Dropping NETIF_F_SG since no checksum feature. > > > > On Mon, 9 Oct 2006 19:47:05 +0200 > > "Michael S. Tsirkin" <mst@mellanox.co.il> 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
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