netdev - Re: [PATCH v2 2/4] Defer skb allocation -- new skb_set calls & chain pages in virtio

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Date:	Mon, 14 Dec 2009 17:24:48 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>, Avi Kivity <avi@...hat.com>,
	netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [PATCH v2 2/4] Defer skb allocation -- new skb_set calls & chain pages in virtio_net

On Fri, 11 Dec 2009 11:13:02 pm Shirley Ma wrote:
> Signed-off-by: Shirley Ma <xma@...ibm.com>

I don't think there's a good way of splitting this change across multiple
patches.  And I don't think this patch will compile; I don't think we can
get rid of trim_pages yet.

We *could* first split the receive paths into multiple parts as you have
(eg. add_recvbuf_big etc), then actually rewrite them, but that's too much
work to refactor the code twice.  

So just roll all the driver changes into one patch; so you will have two
patches: one which creates the destroy_bufs API for virtio, and one which
uses it in the virtio_net driver.

> +static struct sk_buff *skb_goodcopy(struct virtnet_info *vi, struct page **page,
> +				    unsigned int *len)

This actually allocates an skb, and transfers the first page to it (via copy
and possibly the first fragment).  skb_from_page() perhaps?

> +		hdr_len = sizeof(hdr->hdr);
> +		offset = sizeof(struct padded_vnet_hdr);
> +	}
> +
> +	memcpy(hdr, p, hdr_len);
> +
> +	*len -= hdr_len;

Perhaps you should return NULL here as an error if *len was < hdr_len?

Thanks,
Rusty.
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