Re: [PATCH bpf-next 6/7] libbpf: BPF Static Keys support

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2023-12-08 at 14:04 -0800, Andrii Nakryiko wrote:
[...]
> > However, if this is the only API we provide, then this makes user's life
> > hard, as they will have to keep track of ids, and programs used, and
> > mapping from "global" id to local ids for each program (when multiple
> > programs use the same static key, which is desirable). If we keep the
> > higher-level "map API", then this simplifies user's life: on a program load
> > a user can send a list of (local_id -> map) mappings, and then toggle all
> > the branches controlled by "a [global] static key" by either
> > 
> >     bpf(MAP_UPDATE_ELEM, map, value)
> > 
> > or
> > 
> >     kfunc bpf_static_key_set(map, value)
> > 
> > whatever is more useful. (I think that keeping the bpf(2) userspace API is
> > worth doing it, as otherwise this, again, makes life harder: users would
> > have to recompile/update iterator programs if new programs using a static
> > key are added, etc.)
> > 
> > Libbpf can simplify life even more by automatically allocating local ids
> > and passing mappings to kernel for a program from the
> > `bpf_static_branch_{unlikely,likely}(&map)`, so that users don't ever thing
> > about this, if don't want to. Again, no relocations are required here.
> > 
> > So, to summarize:
> > 
> >   * A new instruction BPF_JA_CFG[ID,FLAGS,OFFSET] where ID is local to the
> >     program, FLAGS is 0/1 for normal/inverse branches
> > 
> 
> +1 for a dedicated instruction

fwiw, if relocations are used instead of IDs the new instruction does
not have to be a control flow. It might be a mov that sets target
register to a value that verifier treats as unknown. At runtime this
mov could be patched to assign different values. Granted it would be
three instructions:

  mov rax, 0;
  cmp rax, 0;
  je ...
  
instead of one, but I don't believe there would noticeable performance
difference. On a plus side: even simpler verification, likely/unlikely
for free, no need to track if branch is inverted.