netdev - Re: virtio-net header alignment

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Message-Id: <1244743266.9669.64.camel@w-sridhar.beaverton.ibm.com>
Date:	Thu, 11 Jun 2009 11:01:06 -0700
From:	Sridhar Samudrala <sri@...ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Mark McLoughlin <markmc@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Danielius Kudinskas <kudinskas@...ibm.com>,
	aliguori@...ux.vnet.ibm.com, Paul T Knowles <pablo@...ibm.com>,
	netdev@...r.kernel.org
Subject: Re: virtio-net header alignment

On Wed, 2009-06-10 at 15:12 +1000, Herbert Xu wrote:
> On Mon, May 25, 2009 at 05:17:03PM +0100, Mark McLoughlin wrote:
> > 
> > Perhaps; would padding struct virtio_net_hdr_mrg_rxbuf have avoided it?
> > 
> > Cc-ing Herbert, he initially wrote the guest side of this.
> 
> Does this fix it?
> 
> virtio_net: Fix IP alignment on mergeable RX path
> 
> We need to enforce the IP alignment on the mergeable RX path just
> like the other RX path.  Not doing so results in misaligned IP
> headers.

I think this fixes the old RX path rather than the mergeable RX path.
try_fill_recv_maxbufs() is called from try_fill_recv()

        if (!vi->mergeable_rx_bufs) {
                try_fill_recv_maxbufs(vi);
                return;
        }

-Sridhar

> 
> Signed-off-by: Herbert Xu <herbert@...dor.apana.org.au>
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 4d1d479..52fba8e 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -283,10 +283,11 @@ static void try_fill_recv_maxbufs(struct virtnet_info *vi)
>  	for (;;) {
>  		struct virtio_net_hdr *hdr;
> 
> -		skb = netdev_alloc_skb(vi->dev, MAX_PACKET_LEN);
> +		skb = netdev_alloc_skb(vi->dev, MAX_PACKET_LEN + NET_IP_ALIGN);
>  		if (unlikely(!skb))
>  			break;
> 
> +		skb_reserve(skb, NET_IP_ALIGN);
>  		skb_put(skb, MAX_PACKET_LEN);
> 
>  		hdr = skb_vnet_hdr(skb);
> 
> Cheers,

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