Subject: Re: [PATCH bpf-next 05/11] bpf: avoid retpoline for lookup/update/delete calls on maps

From: Jesper Dangaard Brouer <[email-masked]>
Date: Tue, 29 May 2018 19:23:24 +0200

On Mon, 28 May 2018 02:43:38 +0200
Daniel Borkmann <[email-masked]> wrote:

> While some of the BPF map lookup helpers provide a ->map_gen_lookup()
> callback for inlining the map lookup altogether it is not available
> for every map, so the remaining ones have to call bpf_map_lookup_elem()
> helper which does a dispatch to map->ops->map_lookup_elem(). In
> times of retpolines, this will control and trap speculative execution
> rather than letting it do its work for the indirect call and will
> therefore cause a slowdown. Likewise, bpf_map_update_elem() and
> bpf_map_delete_elem() do not have an inlined version and need to call
> into their map->ops->map_update_elem() resp. map->ops->map_delete_elem()
> handlers.
> 
> Before:
> 
>   # bpftool p d x i 1

I would really appreciate if we can use the long options in these kind
of examples.  It makes the command "self-documenting" and searchable by
google.

Here it would be:

 # bpftool prog dump xlated id 1

>     0: (bf) r2 = r10
>     1: (07) r2 += -8
>     2: (7a) *(u64 *)(r2 +0) = 0
>     3: (18) r1 = map[id:1]
>     5: (85) call __htab_map_lookup_elem#232656
>     6: (15) if r0 == 0x0 goto pc+4
>     7: (71) r1 = *(u8 *)(r0 +35)
>     8: (55) if r1 != 0x0 goto pc+1
>     9: (72) *(u8 *)(r0 +35) = 1
>    10: (07) r0 += 56
>    11: (15) if r0 == 0x0 goto pc+4
>    12: (bf) r2 = r0
>    13: (18) r1 = map[id:1]
>    15: (85) call bpf_map_delete_elem#215008  <-- indirect call via
>    16: (95) exit                                 helper
> 
> After:
> 
>   # bpftool p d x i 1

Same here

>     0: (bf) r2 = r10
>     1: (07) r2 += -8
>     2: (7a) *(u64 *)(r2 +0) = 0
>     3: (18) r1 = map[id:1]
>     5: (85) call __htab_map_lookup_elem#233328
>     6: (15) if r0 == 0x0 goto pc+4
>     7: (71) r1 = *(u8 *)(r0 +35)
>     8: (55) if r1 != 0x0 goto pc+1
>     9: (72) *(u8 *)(r0 +35) = 1
>    10: (07) r0 += 56
>    11: (15) if r0 == 0x0 goto pc+4
>    12: (bf) r2 = r0
>    13: (18) r1 = map[id:1]
>    15: (85) call htab_lru_map_delete_elem#238240  <-- direct call
>    16: (95) exit
> 

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer