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Date:	Thu, 03 Apr 2014 14:23:38 -0400 (EDT)
From:	David Miller <>
Subject: Re: [PATCH net-next v3 2/2] xen-netback: Grant copy the header
 instead of map and memcpy

From: Zoltan Kiss <>
Date: Wed, 2 Apr 2014 18:04:58 +0100

> An old inefficiency of the TX path that we are grant mapping the first slot,
> and then copy the header part to the linear area. Instead, doing a grant copy
> for that header straight on is more reasonable. Especially because there are
> ongoing efforts to make Xen avoiding TLB flush after unmap when the page were
> not touched in Dom0. In the original way the memcpy ruined that.
> The key changes:
> - the vif has a tx_copy_ops array again
> - xenvif_tx_build_gops sets up the grant copy operations
> - we don't have to figure out whether the header and first frag are on the same
>   grant mapped page or not
> Note, we only grant copy PKT_PROT_LEN bytes from the first slot, the rest (if
> any) will be on the first frag, which is grant mapped. If the first slot is
> smaller than PKT_PROT_LEN, then we grant copy that, and later __pskb_pull_tail
> will pull more from the frags (if any)
> Signed-off-by: Zoltan Kiss <>

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