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Date:	Mon, 09 Apr 2007 11:59:21 -0700 (PDT)
From:	David Miller <>
Subject: Re: [NET]: Use csum_start offset instead of skb_transport_header

From: Herbert Xu <>
Date: Wed, 4 Apr 2007 12:05:30 +1000

> Hi Dave:
> I've finally managed to kill the Xen csum_blank/proto_data_valid hack.
> In the process I needed to make two more changes to Linux.  Here are
> the patches against net-2.6.22.
> [NET]: Use csum_start offset instead of skb_transport_header
> The skb transport pointer is currently used to specify the start
> of the checksum region for transmit checksum offload.  Unfortunately,
> the same pointer is also used during receive side processing.
> This creates a problem when we want to retransmit a received
> packet with partial checksums since the skb transport pointer
> would be overwritten.
> This patch solves this problem by creating a new 16-bit csum_start
> offset value to replace the skb transport header for the purpose
> of checksums.  This offset is calculated from skb->head so that
> it does not have to change when skb->data changes.
> No extra space is required since csum_offset itself fits within
> a 16-bit word so we can use the other 16 bits for csum_start.
> For backwards compatibility, just before we push a packet with
> partial checksums off into the device driver, we set the skb
> transport header to what it would have been under the old scheme.
> Signed-off-by: Herbert Xu <>

Applied, nice work Herbert.
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