On 16-02-23 02:03 PM, John Fastabend wrote:
In the initial implementation the only way to stop a rule from being
inserted into the hardware table was via the device feature flag.
However this doesn't work well when working on an end host system
where packets are expect to hit both the hardware and software
datapaths.
For example we can imagine a rule that will match an IP address and
increment a field. If we install this rule in both hardware and
software we may increment the field twice. To date we have only
added support for the drop action so we have been able to ignore
these cases. But as we extend the action support we will hit this
example plus more such cases. Arguably these are not even corner
cases in many working systems these cases will be common.
To avoid forcing the driver to always abort (i.e. the above example)
this patch adds a flag to add a rule in software only. A careful
user can use this flag to build software and hardware datapaths
that work together. One example we have found particularly useful
is to use hardware resources to set the skb->mark on the skb when
the match may be expensive to run in software but a mark lookup
in a hash table is cheap. The idea here is hardware can do in one
lookup what the u32 classifier may need to traverse multiple lists
and hash tables to compute. The flag is only passed down on inserts
on deletion to avoid stale references in hardware we always try
to remove a rule if it exists.
Notice we do not add a hardware only case here. If you were to
add a hardware only case then you are stuck with the problem
of where to stick the software representation of that filter
rule. If its stuck on the same filter list as the software only and
software/hardware rules it then has to be walked over and ignored
in the classify path. The overhead is not huge but is measurable.
And with so much work being invested in speeding up rx/tx of
pkt processing this is unacceptable IMO. The other option is to
have a special hook just for hardware only resources. This is
implemented in the next patch.
Dont have much time to look closely - will do later. Just wanted
to quip:
Would it make sense to have a user flag which says, "please store
this in s/ware - dont use it in s/ware just install it in h/ware."
This should be totally optional policy, but would help find the rules
faster from a control plane if i look for them in s/ware first.
There's some really freaking slow hardware interfaces out there...
(a record of 60 seconds to find something is not unheard of).
cheers,
jamal
