Re: [Xen-devel] [PATCH net-next RFC 3/5] xen-netback: Remove oldTX grant copy definitons

From: Zoltan Kiss
Date: Thu Oct 31 2013 - 15:46:45 EST

On 30/10/13 09:39, Jan Beulich wrote:
On 30.10.13 at 01:50, Zoltan Kiss <zoltan.kiss@xxxxxxxxxx> wrote:
These became obsolate with grant mapping.

I didn't look at this in detail, but I'm surprised you can get away
without any copying: For one, the header part needs copying
anyway, so you'd pointlessly map and then copy it if it's small
Yep, that's a further plan for optimization. I think I will add that as a separate patch to the series later. But that doesn't necessarily needs these definitions, let's see that later.

And second you need to be prepared for the frontend
to hand you more slots than you can fit in MAX_SKB_FRAGS
which you can't handle with mapping alone afaict.
Oh, I was not aware of this problem. And indeed, the trivial solution is to keep the grant copy methods for such kind of packets, however that sounds quite nasty.
My another idea is to use skb_shinfo(skb)->frag_list for such packets, so the stack will see it as a fragmented IP packet. It might be less efficient than coalescing them into one skb during grant copy at first place, but probably a cleaner solution. If we don't care that much about the performance of such guests, it might be a better solution.
But I don't know that closely the IP fragmentation ideas, so it might be a bad idea. I'm happy to hear comments from people who have more experience with that.

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