Re: [Xen-devel] netchannel vs MAX_SKB_FRAGS (Was: Re: [PATCH] xen/netfront: handle compound page fragments on transmit)

[James Harper <james.harper@xxxxxxxxxxxxxxxx>]

> 
> On 2012-11-21 0:04, Ian Campbell wrote:
> > Limiting to just xen-devel folks.
> >
> > On Tue, 2012-11-20 at 15:54 +0000, Ian Campbell wrote:
> >
> > This highlights a couple of issues, the main one is that implicitly
> > including MAX_SKB_FRAGS in the PV net protocol is just madness. It can
> > and has changed in the past. Someone really needs to (retroactively)
> > figure out what a sensible default minimum which front and back must
> > support is and use it in the front and backends instead of
> > MAX_SKB_FRAGS. Probably something derived from the existing 64K limit
> > (Linux has used 17 and 18 as MAX_SKB_FRAGS in the past).
> >
> > Then perhaps implement a feature-flag to allow front and backends to
> > negotiate something bigger if they like.
> >
> > It might also be interesting for front and backends to coalesce
> > multiple ring slots into compound pages.
> 
> Yes, MAX_SKB_FRAGS is max frag number, not max slot number required.
> But I am not so clear about the whole implementation here. Does it mean
> netfront/netback uses something like MAX_SKB_FRAGS*SLOTS_PER_FRAG?
> and SLOTS_PER_FRAG can be adjusted based on different situation? If
> netfront and netback negotiate this value, then xenstore key feature is
> required, right?
> As mentioned in another compound page fragments patch, for non-
> compound page, this value could be 64K/PAGE_SIZE + 1, but if taking account
> into compound page, the worse case is around 48 slots(in 1+4096+1 case). It
> is hard to negotiate this between netfront and netback for different packets.
> 

FWIW, Windows appears to have no real limit in its SG list, and if its >18 then 
netback (or linux?) just drops such packets. For a 64kb "large" packet, the 
buffer arrangement would likely be eth hdr + ip hdr + tcp hdr + (64K - total 
header size)/PAGE_SIZE + 1 frags. It would be good if the documentation of the 
protocol defined this limitation (which may not even exist if dom0 was 
something other than Linux anyway).

James

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