netdev - Re: [6/6] [VIRTIO] net: Allow receiving SG packets

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Message-ID: <20080422011354.GB14994@gondor.apana.org.au>
Date:	Tue, 22 Apr 2008 09:13:55 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	David Miller <davem@...emloft.net>
Cc:	rusty@...tcorp.com.au, virtualization@...ts.linux-foundation.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org
Subject: Re: [6/6] [VIRTIO] net: Allow receiving SG packets

On Mon, Apr 21, 2008 at 01:04:18PM -0700, David Miller wrote:
> From: Rusty Russell <rusty@...tcorp.com.au>
> Date: Tue, 22 Apr 2008 05:06:16 +1000
> 
> > I'm not sure what the right number is here.  Say worst case is header which 
> > goes over a page boundary then MAX_SKB_FRAGS in the skb, but for some reason 
> > that already has a +2:

The +2 (i.e., extra +1) is for the virtio GSO header.
 
> skb->data is linear, therefore it's not possible to need
> more than one scatterlist entry for it.

Theoretically yes :) But for virtualisation the underlying transport
may present meta-physically contiguous memory that is physically
discrete.  So we may actually need to have multiple SG entries for
skb->data.  However, no current code path should generate packets
with both long skb->data areas *and* skb page frags so we could
just drop them if they show up.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
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