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

[Michel Machado <michel@digirati.com.br>]

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.

    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/95d1d6e8201861a0d0c698bfd06ad606674f1e07/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.