Re: [RFC bpf-next 0/4] Add XDP rx hw hints support performing XDP_REDIRECT

[Lorenzo Bianconi <lorenzo@kernel.org>]

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> On Mon Sep 30, 2024 at 1:49 PM CEST, Lorenzo Bianconi wrote:
> > > Lorenzo Bianconi <lorenzo@kernel.org> writes:
> > > 
> > > >> > We could combine such a registration API with your header format, so
> > > >> > that the registration just becomes a way of allocating one of the keys
> > > >> > from 0-63 (and the registry just becomes a global copy of the header).
> > > >> > This would basically amount to moving the "service config file" into the
> > > >> > kernel, since that seems to be the only common denominator we can rely
> > > >> > on between BPF applications (as all attempts to write a common daemon
> > > >> > for BPF management have shown).
> > > >> 
> > > >> That sounds reasonable. And I guess we'd have set() check the global
> > > >> registry to enforce that the key has been registered beforehand?
> > > >> 
> > > >> >
> > > >> > -Toke
> > > >> 
> > > >> Thanks for all the feedback!
> > > >
> > > > I like this 'fast' KV approach but I guess we should really evaluate its
> > > > impact on performances (especially for xdp) since, based on the kfunc calls
> > > > order in the ebpf program, we can have one or multiple memmove/memcpy for
> > > > each packet, right?
> > > 
> > > Yes, with Arthur's scheme, performance will be ordering dependent. Using
> > > a global registry for offsets would sidestep this, but have the
> > > synchronisation issues we discussed up-thread. So on balance, I think
> > > the memmove() suggestion will probably lead to the least pain.
> > > 
> > > For the HW metadata we could sidestep this by always having a fixed
> > > struct for it (but using the same set/get() API with reserved keys). The
> > > only drawback of doing that is that we statically reserve a bit of
> > > space, but I'm not sure that is such a big issue in practice (at least
> > > not until this becomes to popular that the space starts to be contended;
> > > but surely 256 bytes ought to be enough for everybody, right? :)).
> >
> > I am fine with the proposed approach, but I think we need to verify what is the
> > impact on performances (in the worst case??)
> 
> If drivers are responsible for populating the hardware metadata before
> XDP, we could make sure drivers set the fields in order to avoid any
> memove() (and maybe even provide a helper to ensure this?).

nope, since the current APIs introduced by Stanislav are consuming NIC
metadata in kfuncs (mainly for af_xdp) and, according to my understanding,
we want to add a kfunc to store the info for each NIC metadata (e.g rx-hash,
timestamping, ..) into the packet (this is what Toke is proposing, right?).
In this case kfunc calling order makes a difference.
We can think even to add single kfunc to store all the info for all the NIC
metadata (maybe via a helping struct) but it seems not scalable to me and we
are losing kfunc versatility.

Regards,
Lorenzo

> 
> > > > Moreover, I still think the metadata area in the xdp_frame/xdp_buff is not
> > > > so suitable for nic hw metadata since:
> > > > - it grows backward 
> > > > - it is probably in a different cacheline with respect to xdp_frame
> > > > - nic hw metadata will not start at fixed and immutable address, but it depends
> > > >   on the running ebpf program
> > > >
> > > > What about having something like:
> > > > - fixed hw nic metadata: just after xdp_frame struct (or if you want at the end
> > > >   of the metadata area :)). Here he can reuse the same KV approach if it is fast
> > > > - user defined metadata: in the metadata area of the xdp_frame/xdp_buff
> > > 
> > > AFAIU, none of this will live in the (current) XDP metadata area. It
> > > will all live just after the xdp_frame struct (so sharing the space with
> > > the metadata area in the sense that adding more metadata kv fields will
> > > decrease the amount of space that is usable by the current XDP metadata
> > > APIs).
> > > 
> > > -Toke
> > > 
> >
> > ah, ok. I was thinking the proposed approach was to put them in the current
> > metadata field.
> 
> I've also been thinking of putting this new KV stuff at the start of the
> headroom (I think that's what you're saying Toke?). It has a few nice
> advantanges:
> 
> * It coexists nicely with the current XDP / TC metadata support.
> Those users won't be able to overwrite / corrupt the KV metadata.
> KV users won't need to call xdp_adjust_meta() (which would be awkward -
> how would they know how much space the KV implementation needs).
> 
> * We don't have to move all the metadata everytime we call
> xdp_adjust_head() (or the kernel equivalent).
> 
> Are there any performance implications of that, e.g. for caching?
> 
> This would also grow "upwards" which is more natural, but I think 
> either way the KV API would hide whether it's downwards or upwards from
> users.
> 
> >
> > Regards,
> > Lorenzo
> 

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