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

[ANNIE LI <annie.li@xxxxxxxxxx>]

On 2012-11-23 10:10, James Harper wrote:
> > 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.
This should be netback dropping packets in function netbk_count_requests,

                if (unlikely(frags >= MAX_SKB_FRAGS)) {
                        netdev_dbg(vif->dev, "Too many frags\n");
                        return -frags;
                }

> 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).
Agree.

Thanks
Annie
> James
>
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