Re: [dpdk-dev] [PATCH v2 1/3] ethdev: add actions to modify TCP header fields

[Dekel Peled <dekelp@mellanox.com>]

Thanks, PSB.

> -----Original Message-----
> From: Adrien Mazarguil <adrien.mazarguil@6wind.com>
> Sent: Thursday, April 4, 2019 4:26 PM
> To: Ori Kam <orika@mellanox.com>
> Cc: Dekel Peled <dekelp@mellanox.com>; wenzhuo.lu@intel.com;
> jingjing.wu@intel.com; bernard.iremonger@intel.com; Yongseok Koh
> <yskoh@mellanox.com>; Shahaf Shuler <shahafs@mellanox.com>;
> dev@dpdk.org
> Subject: Re: [PATCH v2 1/3] ethdev: add actions to modify TCP header fields
> 
> Hi Ori,
> 
> (trimming message down a bit)
> 
> On Thu, Apr 04, 2019 at 09:01:52AM +0000, Ori Kam wrote:
> > Hi Adrien,
> >
> > PSB
> <snip>
> >
> > > From: Adrien Mazarguil <adrien.mazarguil@6wind.com>
> <snip>
> > > On Wed, Apr 03, 2019 at 10:49:09AM +0000, Dekel Peled wrote:
> > > > Thanks, PSB.
> <snip>
> > > > > From: Adrien Mazarguil <adrien.mazarguil@6wind.com>
> <snip>
> > > > > I still don't agree with the wording as it implies one must
> > > > > combine this
> > > action
> > > > > with the TCP pattern item or else, while one should simply
> > > > > ensure the presence of TCP traffic somehow. This may be done by a
> prior filtering rule.
> > > > >
> > > > > So here's a generic suggestion which could be used with pretty
> > > > > much all modifying actions (other actions have the same problem
> > > > > and will have to be fixed as well eventually):
> > > > >
> > > > >  Using this action on non-matching traffic results in undefined
> behavior.
> > > > >
> > > > > This comment applies to all instances in this patch.
> > > >
> > > > I accept your suggestion, indeed the existing actions have the
> > > > problematic
> > > condition.
> > > > However I would like to currently leave this patch as-is for consistency.
> > > > I will send a fix patch for next release, applying the updated
> > > > text to all
> > > modify-header actions.
> > >
> > > Please do it now as it's much more difficult to change an existing
> > > API later (think deprecation notices and endless discussions); even
> > > seemingly minor documentation issues like this one may affect
> applications.
> > >
> > I agree that changing API is not easy. This is why I think we should
> > keep Dekel patch, there is a number of API and consistency is very
> > important. Also the PMD is based on the current description that such
> command should fail.
> >
> > So lets keep it this way if you want to change all API then and only then this
> API should be changed.
> 
> Wait, I'm not asking Delek to modify existing code/APIs right now, only to

It's Dekel, not Delek (nor any other permutation of these letters).

> document these new actions properly from the start so we don't have to do
> it later (you even acknowledged it's more difficult that way).
> 
> So I fail to understand why it's so important for their documentation to be
> consistent with unrelated and badly documented actions?
> 
> Note the change I'm asking for at the API level doesn't affect PMD code,
> which remains free to put extra limitations (namely the presence of TCP
> pattern items). It's just that these limitations have nothing to do in the API
> itself.

Accepted, I will change the documentation as you suggested, with note that the resulting undefined behavior is per PMD implementation.

Regarding Andrew's suggestion: "Shouldn't these action be RTE_FLOW_ACTION_TYPE_MOD_TCP_{ACK,SEQ} with singed 32-bit integer parameter (negative to decrement, positive to increment)?"
I will leave the actions as is, the action names indicate the operation they perform. 

> 
> <snip>
> > > > It's either 2 actions with 1 parameters, or 1 action with 2 parameters.
> > > > The current implementation is more straight-forward in my opinion.
> > >
> > > I generally also prefer the one action per thing to do approach, but
> > > seeing the kind of actions you're adding, I fear we'll soon end up
> > > with lots of similar rte_flow_action_* structures modifying a single
> > > 32-bit value in some way.
> > >
> > > So for the same reasons as above, I think it's the right time to
> > > define a shared structure to rule them all, or maybe even let users
> > > provide a rte_be32_t/uint32_t/whatever pointer directly as a conf
> > > pointer (not as straightforward to document though).
> > >
> > > An object to rule them all would look something like that:
> > >
> > >  union rte_flow_integer {
> > >      rte_be64_t be64;
> > >      rte_le64_t le64;
> > >      uint64_t u64;
> > >      int64_t i64;
> > >      rte_be32_t be32;
> > >      rte_le32_t le32;
> > >      uint32_t u32;
> > >      int32_t i32;
> > >      uint8_t u8;
> > >      int8_t i8;
> > >  };
> > >
> > > Then actions that need a single integer value only have to document
> > > which field is relevant to them. How about that?
> > >
> >
> > Like my previous comment. I understand your idea, but it has no huge
> > advantage compared to the suggested one by Dekel which also match all
> other API.
> >
> > Currently for each action we have a direct command, which is easy to
> understand by using your idea we break this concept.
> 
> Yes, although not all actions have a configuration structure. Those that do
> indeed have a rte_flow_action_* counterpart, but it doesn't have to be
> unique, see RTE_FLOW_ITEM_GTP/GTPC/GTPU for instance.
> 
> Likewise this patch adds struct rte_flow_action_modify_tcp_seq shared by
> RTE_FLOW_ACTION_TYPE_INC_TCP_SEQ and
> RTE_FLOW_ACTION_TYPE_DEC_TCP_SEQ although they lack a common
> prefix (inc_tcp/dec_tcp vs. modify_tcp). The type to use is covered by
> documentation and that's fine.
> 
> So why not go a little further and share the exact same structure with
> RTE_FLOW_ACTION_TYPE_INC_TCP_ACK and
> RTE_FLOW_ACTION_TYPE_DEC_TCP_ACK?
> 

Accepted, I will add union as you suggested (plus 16 bit values as Andrew noted) and use it for all the new actions.

> And while there, why not plan for subsequent actions that take a single
> integer value of some kind, because modifying existing APIs once upstream
> is complicated... See where I'm going?
> 
> > There is no issue with having a large number of actions, it is even
> > easer to read and document if each action is dedicated, as you can also see
> from OVS.
> 
> I'm actually fine with a large number of actions (rte_flow can support 2^31
> unique actions). Not so much with a large number of identical configuration
> structures that only differ by name associated with them. This is what I'd like
> to avoid before it's too late.
> 
> > So I vote to keep Dekel patch as is.
> 
> I don't, I guess another vote is needed to decide :)
> 
> --
> Adrien Mazarguil
> 6WIND