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Re: [Xen-devel] [PATCH net v2 1/3] xen-netback: remove pointless clause from if statement

From: Paul Durrant
> > The behaviour of the Windows frontend is different to netfront; it tries to
> > keep the shared ring as full as possible so the estimate could be as
> > pessimistic as you like (as long as it doesn't exceed ring size ;-)) and 
> > you'd
> > never see the lock-up. For some reason (best known to the originator of the
> > code I suspect) the Linux netfront driver limits the number of requests it
> > posts into the shared ring leading to the possibility of lock-up in the case
> > where the backend needs more slots than the fontend 'thinks' it should.
> > But from what i read the ring size is determined by the frontend .. so that 
> > PV
> > driver should be able to guarantee that itself ..
> >
> The ring size is 256 - that's baked in. The number of pending requests 
> available to backend *is*
> determined by the frontend.
> > Which begs for the question .. was that change of max_slots_needed
> > calculation *needed* to prevent the problem you saw on "Windows Server
> > 2008R2",
> > or was that just changed for correctness ?
> >
> It was changed for correctness. As I understand it, use of MAX_SKB_FRAGS is 
> incorrect if compound
> pages are in use as the page size is no longer the slot size. It's also 
> wasteful to always wait for
> space for a maximal packet if the packet you have is smaller so the intention 
> of the max estimate was
> that it should be at least the number of slots required but not excessive. I 
> think you've proved that
> making such an estimate is just too hard and since we don't want to fall back 
> to the old dry-run style
> of slot counting (which meant you had two codepaths that *must* arrive at the 
> same number - and they
> didn't, which is why I was getting the lock-up with Windows guests) I think 
> we should just go with
> full-packing so that we don't need to estimate.

A reasonable high estimate for the number of slots required for a specific
message is 'frag_count + total_size/4096'.
So if that are that many slots free it is definitely ok to add the message.

I can see a more general problem for transmits.
I believe a NAPI driver is supposed to indicate that it can't accept
a tx packet in advance of being given a specific packet to transmit.
This means it has to keep enough tx ring space for a worst case packet
(which in some cases can be larger than 1+MAX_SKB_FRAGS) even though
such a packet is unlikely.
I would be tempted to save the skb that 'doesn't fit' within the driver
rather than try to second guess the number of fragments the next packet
will need.

FWIW the USB3 'bulk' driver has the same problem, fragments can't cross
64k boundaries.


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