netdev - Re: [PATCH net-next] net: netdev_alloc_skb() use build

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Date:	Mon, 04 Jun 2012 16:01:41 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Willy Tarreau <w@....eu>, David Miller <davem@...emloft.net>,
	netdev@...r.kernel.org
Subject: Re: [PATCH net-next] net: netdev_alloc_skb() use build_skb()

On Mon, 2012-06-04 at 16:41 +0300, Michael S. Tsirkin wrote:

> This is generally what virtio does, take a look:
> page_to_skb fills the first fragment and receive_mergeable fills the
> rest (other modes are for legacy hardware).
> 
> The way hypervisor now works is this (we call it mergeable buffers):
> 
> - pages are passed to hardware
> - hypervisor puts virtio specific stuff in first 12 bytes
>   on first page
> - following this, the rest of the first page and all following
>   pages have data
> 
> The driver gets the 1st page, allocates the skb, copies out the 12 byte
> header and copies the first 128 bytes of data into skb.
> The rest if any is populated by the pages.
> 
> So I guess I'm asking for advice, would it make sense to switch to build_skb
> and how best to handle the data copying above? Maybe it would help
> if we changed the hypervisor to write the 12 bytes separately?
>   

Thanks for these details.

Not sure 12 bytes of headroom would be enough (instead of the
NET_SKB_PAD reserved in netdev_alloc_skb_ip_align(), but what could be
done indeed is to use the first page as the skb->head, so using
build_skb() indeed, removing one fragment, one (small) copy and one
{put|get}_page() pair.



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