[HEADS UP!] IPFW Ideas: possible SoC 2008 candidate

Vadim Goncharov vadim_nuclight at mail.ru
Wed Mar 26 23:47:08 PDT 2008

Hi Julian Elischer! 

On Wed, 26 Mar 2008 10:31:12 -0700; Julian Elischer wrote about 'Re: [HEADS UP!] IPFW Ideas: possible SoC 2008 candidate':

>>> here are some of my ideas for ipfw changes:
>>> 1/ redo locking so that packets do not have to get locks on the 
>>> structure... I have several ideas on this
>> Currently the main need for locking arises for rule byte/packet counters. The
>> easiest short-term solution
> The main need for locking is that the rules can be changed while a 
> processor is traversing the rule set.

Oh, I haven't finished the letter. I wanted to say that easiest short-term
solution would be splitting main ruleset lock into 2 locks: one for ruleset and
one for counters, and wrap the only "f->*cnt +=" with that locks.

>>> 2/ allow separate firewalls to be used at different parts of the 
>>> network stack (i.e allow multiple taboe sto co-exist)
> there are many places that ipfw is currently callable from.
> ip_input(), ip_output(), ether_demux(), if_brige, ether_output()

> it would be interesting tobe able to have differnt firewalls in these 
> places (possibly per interface) so that state (e.g. keep_state)
> can be kept seprately for one place then from another.

> for example you may not want the result of 'keep state' on an
> external interface to necessarily affect what happens to
> packets from the same session when viewed traversing an internal 
> interface.

> Currently on my more complex ipfw rule sets I break the rule sets out
> so that packets in different places traverse different rules
> but it would be nice to have it explicitly supported.

Good. That should be added to proposal in part of creation of dynamic rules in
any state: by default all-shared dynamic rules should be used (to preserve POLA),
and an option in rule can be specified for a specific pass/layer.

>> Umm, could you explain it a little?..
>>> 3/ possibly keeping per CPU stats..
>> How that would be represented to user?
> it wouldn't.. you'd add them together before presenting them.
> but every time a packet changes a counter that is shared, there is a 
> chance that it is being altered by another processor, so if you have
> fine grained locking in ipfw, you really should use atomic adds,
> which are slow, or accept possibl collisions (which might be ok)
> but still cause a lot of cross cpu TLB flushing.

Looked at the code... oh... I was wrong above with splitting for two locks into
second for counters. It merely doen't exist and counters can get corrupted just
now :-) The idea to keep separate per-CPU counters *could* be just reasonable,
but as O_COUNTERLIMIT is likely to be introduced, it will need to access to
already combined values, which now can be just for every packet, thus
invalidating whole idea.

The possible solution can again lie in ruleset compiled to single area of
opcodes rather than linked list. In this case counters can be made a separate
from opcodes array of int64's which will be contiguous and much smaller than
current opcodes/counters mix, so chances for cache misses/flushes will be much
smaller too.

WBR, Vadim Goncharov. ICQ#166852181       mailto:vadim_nuclight at mail.ru
[Moderator of RU.ANTI-ECOLOGY][FreeBSD][http://antigreen.org][LJ:/nuclight]

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