Re: [dpdk-dev] [PATCH v2] net/failsafe: fix source port ID in Rx packets

[Thomas Monjalon <thomas@monjalon.net>]

18/04/2019 19:09, Adrien Mazarguil:
> On Thu, Apr 18, 2019 at 06:54:22PM +0200, Thomas Monjalon wrote:
> <snip>
> > > <snip>
> > > > > "slave" is a wording from bonding.
> > > > > In failsafe, it is sub-device, isn't it?
> > > 
> > > I don't mind, although grep shows a couple of comments talking about slaves
> > > already. Either way I think it fits as those are failsafe's pets, as in
> > > failsafe does whatever it wants to them and they don't have a say :)
> > > 
> > > Does it warrant a v3?
> > 
> > Yes please, except if Ferruh is already doing the change on apply.
> 
> Will do.
> 
> <snip>
> > > > > I'm afraid the performance drop to be hard.
> > > 
> > > Mbufs are still hot from the oven at this stage, so it's not *that*
> > > expensive. I don't see a more efficient approach.
> > 
> > Yes, Ali did some quick tests showing no perf drop.
> 
> Great.
> 
> > > > > How the port id in mbuf is used exactly?
> > > 
> > > Applications that dissociate Rx itself from packet processing, or whenever a
> > > networking stack is involved. Basically every time some code wonders where a
> > > packet comes from due to lack of context and looks at m->port for the
> > > answer (e.g. checking that a packet arrives on the right port given its
> > > destination address).
> > > 
> > > > > What crash are you seeing?
> > > 
> > > None, thankfully. In my specific use case, 6WINDGate's stack simply drops
> > > traffic coming from unknown ports.
> > > 
> > > However nothing prevents applications from using m->port as an index of some
> > > array they allocated to quickly retrieve port context without looking it
> > > up. They wouldn't expect indices they do not know about in there; assuming
> > > it will result in a crash is not far fetched.
> > > 
> > > > Another way to fix it without performance drop would be to add
> > > > a new driver op to set the top-level port id.
> > > > This top-level id would be stored in the private structure of the port,
> > > > initialized with the port id of the port itself, and used to fill mbufs.
> > > > 
> > > > Thoughts?
> > > 
> > > Adding a new devop as a fix would be a problem for stable releases, so this
> > > patch is definitely needed, at least as a first step.
> > > 
> > > I'm not against a new API, however would it be worth the trouble? Especially
> > > considering it would only be used by failsafe-like drivers with something to
> > > hide from applications which is not the main use case.
> > > 
> > > For some PMDs, this operation could only be done at init time before port ID
> > > is stored in private Rx queue data for fast retrieval. Retrieving it through
> > > a pointer so it can be updated anytime would make it more expensive than
> > > necessary for them.
> > 
> > I don't understand this comment.
> > The port id is currently retrieved via some pointers already.
> > I suggest to look at private structure, it is not different.
> 
> See "rep->port = rxq->port_id" in mlx4_rxtx.c for instance. Port ID is
> cached in private queue data structure (struct rxq) and retrieved there to
> avoid looking it up in non-local data structure rxq->priv->dev_data->port.
> In fact rxq->priv is not accessed even once during Rx.

OK, thanks for the explanation.

> > > It's understood that having failsafe in the dataplane has a cost, but even
> > > with the proposed fix, that cost is dwarfed by the amount of work done by a
> > > true PMD (and the application) for Rx processing.
> > > 
> > > My suggestion is to wait for someone to complain about the performance
> > > compared to what they had before that fix, only then see what we can do.
> > 
> > OK