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Re: [Xen-devel] Is: SKB_MAX_LEN bites again. Was: Re: bug disabling guest interface

On 2013-3-12 20:18, Wei Liu wrote:
On Tue, 2013-03-12 at 11:40 +0000, Ian Campbell wrote:
On Sun, 2013-03-10 at 19:18 +0000, Wei Liu wrote:
On Sat, Mar 09, 2013 at 02:57:16AM +0000, Ian Campbell wrote:
- change MAX_SKB_FRAGS to 19 to accommodate all guests
Changing MAX_SKB_FRAGS is *not* an option upstream. This might be a
useful local hack but we need to drop the idea as a long term fix.

Ugh. The negotiations between host and guest is probably the best
choice. The issues you are going to hit are that you might need
to redo the skbs to match what the frontend's max is.
IMHO the right fix is for netback to coalesce as it copies from the
frontend if it needs to do so, it is copying anyway so it should be
cheap enough. I thought we had discussed this and someone was working on
implementing it. If not Annie then perhaps it was Matt or Siva (both now

As a short term fix, can we use skb_linearize() if skb->nr_frags>=
No, because that would require changing the frontend, while this fix
needs to be in the backend if older guests are to continue working.

You can't use skb_linearize in netback as is because you would first
need to be able to build the skb with nr_frags>= MAX_SKB_FRAGS in order
to pass it to that function.

Yes, the idea is to define NETBK_MAX_SKB_FRAGS to some bigger number
(say 20) to accommodate the possible maximum number of frags in
frontend. The thing that truly matters it the skb->len, which should be
<64K, nr_frags is not important.

I doubt this would work since you can not build out skb with nr_frags >= MAX_SKB_FRAGS. See following code in skbuff.h,

#if (65536/PAGE_SIZE + 1) < 16
#define MAX_SKB_FRAGS 16UL
#define MAX_SKB_FRAGS (65536/PAGE_SIZE + 1)

and every skb contains MAX_SKB_FRAGS frags,

struct skb_shared_info {
skb_frag_t      frags[MAX_SKB_FRAGS];

Coalescing frags before building skb could avoid this issue.


I wrote some code during weekend and it seemed to work, but I have not
verify it extensively.


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