Re: [dpdk-dev] [PATCH 2/2] lpm: hide internal data

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>
> 
> Hello,
> 
> On 15/10/2020 18:38, Honnappa Nagarahalli wrote:
> > <snip>
> >>
> >> On 10/14/20 7:57 PM, Honnappa Nagarahalli wrote:
> >>>>>> On 13/10/2020 18:46, Michel Machado wrote:
> >>>>>>> On 10/13/20 11:41 AM, Medvedkin, Vladimir wrote:
> >>>>>>>> Hi Michel,
> >>>>>>>>
> >>>>>>>> Could you please describe a condition when LPM gets inconsistent?
> >>>>>>>> As I can see if there is no free tbl8 it will return -ENOSPC.
> >>>>>>>
> >>>>>>>       Consider this simple example, we need to add the following
> >>>>>>> two prefixes with different next hops: 10.99.0.0/16,
> >>>>>>> 18.99.99.128/25. If the LPM table is out of tbl8s, the second
> >>>>>>> prefix is not added and Gatekeeper will make decisions in
> >>>>>>> violation of the policy. The data structure of the LPM table is
> >>>>>>> consistent, but its content inconsistent with the policy.
> >>> max_rules and number_tbl8s in 'struct rte_lpm' contain the config
> >> information. These 2 fields do not change based on the routes added
> >> and do not indicate the amount of space left. So, you cannot use this
> >> information to decide if there is enough space to add more routes.
> 
> Thanks Honnappa, agree, these two fields are read only after LPM
> initialization, I confused them with rte_fib's "rsvd_tbl8s" and "cur_tbl8s", so
> there is no need to read them directly from LPM after initialization. I'd
> suggest just keeping them as external variables outside of the LPM library (in
> kind of a global configuration I suppose?).
> 
> >>
> >>      We are aware that those fields hold the config information not a
> >> status of the LPM table.
> >>
> >>      Before updating a LPM table that holds network prefixes derived
> >> from threat intelligence, we compute the minimum values for max_rules
> >> and number_tbl8s. Here is an example of how we do it:
> >>
> https://github.com/AltraMayor/gatekeeper/blob/95d1d6e8201861a0d0c698
> >> bfd06ad606674f1e07/lua/examples/policy.lua#L135-L166
> >>
> >>      Once these minimum values are available, we get the parameters
> >> of the LPM table to be updated and check if we can update it, or have to
> recreate it.
> >>
> >>>>>> Aha, thanks. So do I understand correctly that you need to add a
> >>>>>> set of routes atomically (either the entire set is installed or nothing)?
> >>>>>
> >>>>>       Yes.
> >>>>>
> >>>>>> If so, then I would suggest having 2 lpm and switching them
> >>>>>> atomically after a successful addition. As for now, even if you
> >>>>>> have enough tbl8's, routes are installed non atomically, i.e.
> >>>>>> there will be a time gap between adding two routes, so in this
> >>>>>> time interval the table will be inconsistent with the policy.
> >>>>>> Also, if new lpm algorithms are added to the DPDK, they won't
> >>>>>> have such a thing as tbl8.
> >>>>>
> >>>>>       Our code already deals with synchronization.
> >>> If the application code already deals with synchronization, is it
> >>> possible to
> >> revert back (i.e. delete the routes that got added so far) when the
> >> addition of the route-set fails?
> >>
> >>      The way the code is structured, this would require a significant
> >> rewrite because the code assumes that it will succeed since the
> >> capacity of the LPM tables was already checked.
> >>
> >>>>>>>> On 13/10/2020 15:58, Michel Machado wrote:
> >>>>>>>>> Hi Kevin,
> >>>>>>>>>
> >>>>>>>>>       We do need fields max_rules and number_tbl8s of struct
> >>>>>>>>> rte_lpm, so the removal would force us to have another patch
> >>>>>>>>> to our local copy of DPDK. We'd rather avoid this new local
> >>>>>>>>> patch because we wish to eventually be in sync with the stock
> DPDK.
> >>>>>>>>>
> >>>>>>>>>       Those fields are needed in Gatekeeper because we found a
> >>>>>>>>> condition in an ongoing deployment in which the entries of
> >>>>>>>>> some LPM tables may suddenly change a lot to reflect policy
> changes.
> >>>>>>>>> To avoid getting into a state in which the LPM table is
> >>>>>>>>> inconsistent because it cannot fit all the new entries, we
> >>>>>>>>> compute the needed parameters to support the new entries,
> and
> >>>>>>>>> compare with the current parameters. If the current table
> >>>>>>>>> doesn't fit everything, we have to replace it with a new LPM
> table.
> >>>>>>>>>
> >>>>>>>>>       If there were a way to obtain the struct rte_lpm_config
> >>>>>>>>> of a given LPM table, it would cleanly address our need. We
> >>>>>>>>> have the same need in IPv6 and have a local patch to work
> >>>>>>>>> around it (see
> >>>>>>>>>
> >>>>
> >>
> https://github.com/cjdoucette/dpdk/commit/3eaf124a781349b8ec8cd880db
> >>>> 26a78115cb8c8f).
> >>> I do not see why such an API is not possible, we could add one API
> >>> that
> >> returns max_rules and number_tbl8s (essentially, the config that was
> >> passed to rte_lpm_create API).
> >>> But, is there a possibility to store that info in the application as
> >>> that data
> >> was passed to rte_lpm from the application?
> >>
> >>      A suggestion for what this API could look like:
> >>
> >> void rte_lpm_get_config(const struct rte_lpm *lpm, struct
> >> rte_lpm_config *config); void rte_lpm6_get_config(const struct
> >> rte_lpm6 *lpm, struct rte_lpm6_config *config);
> >>
> >>      If the final choice is for not supporting a way to retrieve the
> >> config information on the API, we'll look for a place to keep a copy
> >> of the parameters in our code.
> > IMO, this is not a performance critical path and it is not a difficult solution to
> store these values in the application. My suggestion is to skip adding the API
> and store the values in the application.
> > Vladimir, what's your opinion?
> 
> Agree. Global vars or part of a global configuration could be used here.
Thank you. I think we are fine to go ahead with merging this patch.

> 
> >
> 
> --
> Regards,
> Vladimir