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

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Message-ID: <1338864631.2760.1971.camel@edumazet-glaptop>
Date:	Tue, 05 Jun 2012 04:50:31 +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 Tue, 2012-06-05 at 00:20 +0300, Michael S. Tsirkin wrote:
> On Mon, Jun 04, 2012 at 09:56:52PM +0200, Eric Dumazet wrote:
> > On Mon, 2012-06-04 at 22:48 +0300, Michael S. Tsirkin wrote:
> > 
> > > If I do this what will truesize be? 128, no?
> > 
> > My example was not correct, since you must have enough room for the
> > SKB_DATA_ALIGN(sizeof(struct skb_shared_info))  ( 320 bytes )
> > 
> > So it would be 128 + 320 = 448
> > 
> 
> 
> Ugh. I forgot about that. shinfo goes into the same page,
> so we'll have to also make all frags shorter by 320
> to leave space for shinfo at tail.
> overall looks like we need hyprevisor extensions if
> we want to use build_skb ...

Maybe not.

If you provided a 2048 bytes block and hypervisor filled one (small)
frame, there might be available room at the end of the block for the
shinfo already.

If yes : You can use build_skb()
If not : netdev_alloc_skb_ip_align()

I mentioned this trick for ixgbe driver.


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