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Message-Id: <20061010.203624.91207079.davem@davemloft.net> Date: Tue, 10 Oct 2006 20:36:24 -0700 (PDT) From: David Miller <davem@...emloft.net> To: rdreier@...co.com Cc: mst@...lanox.co.il, shemminger@...l.org, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, openib-general@...nib.org, rolandd@...co.com Subject: Re: Dropping NETIF_F_SG since no checksum feature. From: Roland Dreier <rdreier@...co.com> Date: Tue, 10 Oct 2006 20:33:46 -0700 > Michael> My guess was, an extra pass over data is likely to be > Michael> expensive - dirtying the cache if nothing else. But I do > Michael> plan to measure that, and see. > > I don't get it -- where's the extra pass? If you can't compute the > checksum on the NIC then you have to compute sometime it on the CPU > before passing the data to the NIC. Also, if you don't do checksumming on the card we MUST copy the data (be it from a user buffer, or from a filesystem page cache page) into a private buffer since if the data changes the checksum would become invalid, as I mentioned in another email earlier. Therefore, since we have to copy anyways, it always is better to checksum in parallel with the copy. So the whole idea of SG without hw-checksum support is without much merit at all. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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