netdev - Re: [PATCH 3/3] virtio_net: VIRTIO_NET_F_MSG_RXBUF (imprive rcv buffer allocation)

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Date:	Sun, 16 Nov 2008 22:42:19 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	rusty@...tcorp.com.au
Cc:	netdev@...r.kernel.org, markmc@...hat.com,
	herbert@...dor.apana.org.au
Subject: Re: [PATCH 3/3] virtio_net: VIRTIO_NET_F_MSG_RXBUF (imprive rcv
 buffer allocation)

From: Rusty Russell <rusty@...tcorp.com.au>
Date: Mon, 17 Nov 2008 13:47:42 +1030

> If segmentation offload is enabled by the host, we currently allocate
> maximum sized packet buffers and pass them to the host. This uses up
> 20 ring entries, allowing us to supply only 20 packet buffers to the
> host with a 256 entry ring. This is a huge overhead when receiving
> small packets, and is most keenly felt when receiving MTU sized
> packets from off-host.
> 
> The VIRTIO_NET_F_MRG_RXBUF feature flag is set by hosts which support
> using receive buffers which are smaller than the maximum packet size.
> In order to transfer large packets to the guest, the host merges
> together multiple receive buffers to form a larger logical buffer.
> The number of merged buffers is returned to the guest via a field in
> the virtio_net_hdr.
> 
> Make use of this support by supplying single page receive buffers to
> the host. On receive, we extract the virtio_net_hdr, copy 128 bytes of
> the payload to the skb's linear data buffer and adjust the fragment
> offset to point to the remaining data. This ensures proper alignment
> and allows us to not use any paged data for small packets. If the
> payload occupies multiple pages, we simply append those pages as
> fragments and free the associated skbs.
> 
> This scheme allows us to be efficient in our use of ring entries
> while still supporting large packets. Benchmarking using netperf from
> an external machine to a guest over a 10Gb/s network shows a 100%
> improvement from ~1Gb/s to ~2Gb/s. With a local host->guest benchmark
> with GSO disabled on the host side, throughput was seen to increase
> from 700Mb/s to 1.7Gb/s.
> 
> Based on a patch from Herbert Xu.
> 
> Signed-off-by: Mark McLoughlin <markmc@...hat.com>
> Signed-off-by: Rusty Russell <rusty@...tcorp.com.au> (use netdev_priv)

Applied, but a lot of fuzz and differences when adding to net-next-2.6
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