netdev - Re: [net-next 01/11] ixgbe: Support using build_skb in the case that jumbo frames are disabled

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Date:	Fri, 12 Apr 2013 23:21:59 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Jeff Kirsher <jeffrey.t.kirsher@...el.com>
CC:	<davem@...emloft.net>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	<netdev@...r.kernel.org>, <gospo@...hat.com>, <sassmann@...hat.com>
Subject: Re: [net-next 01/11] ixgbe: Support using build_skb in the case
 that jumbo frames are disabled

On Fri, 2013-04-12 at 04:24 -0700, Jeff Kirsher wrote:
> From: Alexander Duyck <alexander.h.duyck@...el.com>
> 
> This change makes it so that we can enable the use of build_skb for cases
> where jumbo frames are disabled.  The advantage to this is that we do not have
> to perform a memcpy to populate the header and as a result we see a
> significant performance improvement.

I thought about doing this in sfc, but:

[...]
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
[...] 
> +	/* build an skb to go around the page buffer */
> +	skb = build_skb(page_addr, truesize);
> +	if (unlikely(!skb)) {
> +		rx_ring->rx_stats.alloc_rx_buff_failed++;
> +		return NULL;
> +	}
> +
> +	/* we are reusing so sync this buffer for CPU use */
> +	dma_sync_single_range_for_cpu(rx_ring->dev,
> +				      rx_buffer->dma,
> +				      rx_buffer->page_offset,
> +				      ixgbe_rx_bufsz(rx_ring),
> +				      DMA_FROM_DEVICE);
> +
> +	/* update pointers within the skb to store the data */
> +	skb_reserve(skb, NET_IP_ALIGN + NET_SKB_PAD);
> +	__skb_put(skb, size);
> +
> +	if (ixgbe_can_reuse_rx_page(rx_ring, rx_buffer, page, truesize)) {
> +		/* hand second half of page back to the ring */
> +		ixgbe_reuse_rx_page(rx_ring, rx_buffer);
[...]

Suppose this branch is taken, and then:
1. skb is forwarded to another device
2. Packet headers are modified and it's put into a queue
3. Second packet is received into the other half of this page
4. Page cannot be reused, so is DMA-unmapped
5. The DMA mapping was non-coherent, so unmap copies or invalidates
cache

The headers added in step 2 get trashed in step 5, don't they?

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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