linux-kernel - Re: [PATCH v4 09/16] perf tools: Enable indices setting syntax for BPF maps

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Date:	Fri, 11 Dec 2015 09:15:21 -0300
From:	Arnaldo Carvalho de Melo <acme@...nel.org>
To:	Wang Nan <wangnan0@...wei.com>
Cc:	namhyung@...nel.org, linux-kernel@...r.kernel.org,
	pi3orama@....com, mingo@...nel.org, lizefan@...wei.com,
	Alexei Starovoitov <ast@...nel.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
Subject: Re: [PATCH v4 09/16] perf tools: Enable indices setting syntax for
 BPF maps

Em Fri, Dec 11, 2015 at 09:11:45AM -0300, Arnaldo Carvalho de Melo escreveu:
> Em Tue, Dec 08, 2015 at 02:25:37AM +0000, Wang Nan escreveu:
> > This patch introduce a new syntax to perf event parser:
> > 
> >  # perf record -e bpf_file.c/maps.mymap.value[0,3...5,7]=1234/ ...
> 
> Is the above example valid? Wouldn't this be "maps:mymap.value" ?
> 
> > 
> > By utilizing the basic facilities in bpf-loader.c which allow setting
> > different slots in a BPF map separately, the newly introduced syntax
> > allows perf to control specific elements in a BPF map.
> > 
> > Test result:
> > 
> >  # cat ./test_bpf_map_3.c
> >  /************************ BEGIN **************************/
> >  #define SEC(NAME) __attribute__((section(NAME), used))
> >  enum bpf_map_type {
> >      BPF_MAP_TYPE_ARRAY = 2,
> >  };
> >  struct bpf_map_def {
> >      unsigned int type;
> >      unsigned int key_size;
> >      unsigned int value_size;
> >      unsigned int max_entries;
> >  };
> >  static void *(*map_lookup_elem)(struct bpf_map_def *, void *) =
> >      (void *)1;
> >  static int (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =
> >      (void *)6;
> 
> Can you explain the above a bit more? What are the magic 1 and 6 values?

So, from another patch:

 static u64 (*bpf_ktime_get_ns)(void) =
     (void *)5;
 static int (*bpf_trace_printk)(const char *fmt, int fmt_size, ...) =
     (void *)6;
 static int (*bpf_get_smp_processor_id)(void) =
     (void *)8;
 static int (*bpf_perf_event_output)(void *, struct bpf_map_def *, int,
void *, unsigned long) =
     (void *)23;

Where can I get this magical mistery table? Could this be hidden away in
some .h file automagically included in bpf scriptlets so that n00bies
like me don't have to be wtf'ing?

- Arnaldo
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