Re: XDP-hints: Howto support multiple BTF types per packet basis?

[Michal Swiatkowski <michal.swiatkowski@linux.intel.com>]

On Wed, Jun 02, 2021 at 09:18:37AM -0700, Jakub Kicinski wrote:
> On Tue, 01 Jun 2021 17:22:51 -0700 John Fastabend wrote:
> > > If we do this, the BPF program obviously needs to know which fields are
> > > valid and which are not. AFAICT you're proposing that this should be
> > > done out-of-band (i.e., by the system administrator manually ensuring
> > > BPF program config fits system config)? I think there are a couple of
> > > problems with this:
> > > 
> > > - It requires the system admin to coordinate device config with all of
> > >   their installed XDP applications. This is error-prone, especially as
> > >   the number of applications grows (say if different containers have
> > >   different XDP programs installed on their virtual devices).  
> > 
> > A complete "system" will need to be choerent. If I forward into a veth
> > device the orchestration component needs to ensure program sending
> > bits there is using the same format the program installed there expects.
> > 
> > If I tailcall/fentry into another program that program the callee and
> > caller need to agree on the metadata protocol.
> > 
> > I don't see any way around this. Someone has to manage the network.
> 
> FWIW I'd like to +1 Toke's concerns.
> 
> In large deployments there won't be a single arbiter. Saying there 
> is seems to contradict BPF maintainers' previous stand which lead 
> to addition of bpf_links for XDP.
> 
> In practical terms person rolling out an NTP config change may not 
> be aware that in some part of the network some BPF program expects
> descriptor not to contain time stamps. Besides features may depend 
> or conflict so the effects of feature changes may not be obvious 
> across multiple drivers in a heterogeneous environment.
> 
> IMO guarding from obvious mis-configuration provides obvious value.

Hi,

Thanks for a lot of usefull information about CO-RE. I have read
recommended articles, but still don't understand everything, so sorry if
my questions are silly.

As introduction, I wrote small XDP example using CO-RE (autogenerated
vmlinux.h and getting rid of skeleton etc.) based on runqslower
implementation. Offset reallocation of hints works great, I built CO-RE
application, added new field to hints struct, changed struct layout and
without rebuilding application everything still works fine. Is it worth
to add XDP sample using CO-RE in kernel or this isn't good place for
this kind of sample?

First question not stricte related to hints. How to get rid of #define
and macro when I am using generated vmlinux.h? For example I wanted to
use htons macro and ethtype definition. They are located in headers that
also contains few struct definition. Because of that I have redefinition
error when I am trying to include them (redefinition in vmlinux.h and
this included file). What can I do with this besides coping definitions
to bpf code?

I defined hints struct in driver code, is it right place for that? All
vendors will define their own hints struct or the idea is to have one
big hints struct with flags informing about availability of each fields?

For me defining it in driver code was easier because I can have used
module btf to generate vmlinux.h with hints struct inside. However this
break portability if other vendors will have different struct name etc,
am I right?