netdev - RE: [RFC PATCH v7 01/19] Add a new structure for skb buffer from external.

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Date:	Wed, 23 Jun 2010 16:09:40 +0800
From:	"Dong, Eddie" <eddie.dong@...el.com>
To:	"Xin, Xiaohui" <xiaohui.xin@...el.com>,
	Herbert Xu <herbert@...dor.apana.org.au>
CC:	Stephen Hemminger <shemminger@...tta.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mst@...hat.com" <mst@...hat.com>, "mingo@...e.hu" <mingo@...e.hu>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"jdike@...ux.intel.com" <jdike@...ux.intel.com>,
	"Dong, Eddie" <eddie.dong@...el.com>
Subject: RE: [RFC PATCH v7 01/19] Add a new structure for skb buffer from
	external.


> 3) As I have mentioned above, with this idea, netdev_alloc_skb() will
> allocate 
> as usual, the data pointed by skb->data will be copied into the first
> guest buffer. 
> That means we should reserve sufficient room in guest buffer. For PS
> mode 
> supported driver (for example ixgbe), the room will be more than 128.
> After 128bytes, 
> we will put the first frag data. Look into virtio-net.c the function
> page_to_skb() 
> and receive_mergeable(), that means we should modify guest virtio-net
> driver to 
> compute the offset as the parameter for skb_set_frag().
> 
> How do you think about this? Attached is a patch to how to modify the
> guest driver. 
> I reserve 512 bytes as an example, and transfer the header len of the
> skb in hdr->hdr_len. 
> 
Xiaohui & Herbert:
	Mixing copy of head & 0-copy of bulk data imposes additional challange to find the guest buffer. The backend driver may be unable to find a spare guest buffer from virtqueue at that time which may block the receiving process then.
	Can't we completely eliminate netdev_alloc_skb here? Assigning guest buffer at this time makes life much easier.
Thx, Eddie
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