Re: [PATCH bpf-next v5 1/1] bpf, docs: document BPF_MAP_TYPE_ARRAY

[Bagas Sanjaya <bagasdotme@gmail.com>]

On 10/4/22 23:19, Donald Hunter wrote:
> +Examples
> +========
> +
> +Please see the ``tools/testing/selftests/bpf`` directory for functional
> +examples. The sample code below demonstrates API usage.
> +

Since you have many code snippets, better say "The code samples below".

> +Kernel
> +------
> +
> +This snippet shows how to declare an array in a BPF program.
> +
> +.. code-block:: c
> +
> +    struct {
> +            __uint(type, BPF_MAP_TYPE_ARRAY);
> +            __type(key, u32);
> +            __type(value, long);
> +            __uint(max_entries, 256);
> +    } my_map SEC(".maps");
> +
> +
> +This example shows how to access an array element.
> +
> +.. code-block:: c
> +
> +    int bpf_prog(struct __sk_buff *skb)
> +    {
> +            int index = load_byte(skb,
> +                                  ETH_HLEN + offsetof(struct iphdr, protocol));
> +            long *value;
> +
> +            if (skb->pkt_type != PACKET_OUTGOING)
> +                    return 0;
> +
> +            value = bpf_map_lookup_elem(&my_map, &index);
> +            if (value)
> +                    __sync_fetch_and_add(value, skb->len);
> +
> +            return 0;
> +    }
> +
> +Userspace
> +---------
> +
> +BPF_MAP_TYPE_ARRAY
> +~~~~~~~~~~~~~~~~~~
> +
> +This example shows array creation, initialisation and lookup from userspace.
> +

"Initialize the array, set elements, and perform lookup".

> +.. code-block:: c
> +
> +    #include <assert.h>
> +    #include <bpf/libbpf.h>
> +    #include <bpf/bpf.h>
> +
> +    int main(int argc, char **argv)
> +    {
> +	    int fd;
> +	    int ret = 0;
> +	    long value;
> +	    __u32 index = 42;
> +	    __u32 i;
> +
> +	    fd = bpf_map_create(BPF_MAP_TYPE_ARRAY, "example_array",
> +				sizeof(__u32), sizeof(long),
> +				256, 0);
> +	    if (fd < 0)
> +		    return fd;
> +
> +	    /* fill the map with values from 0-255 */
> +	    for (i = 0; i < 256 ; i++) {
> +		    value = i;
> +		    ret = bpf_map_update_elem(fd, &i, &value, BPF_ANY);
> +		    if (ret < 0)
> +			    return ret;
> +	    }
> +
> +	    ret = bpf_map_lookup_elem(fd, &index, &value);
> +	    if (ret < 0)
> +		    return ret;
> +
> +	    assert(value == 42);
> +
> +	    return ret;
> +    }
> +
> +BPF_MAP_TYPE_PERCPU_ARRAY
> +~~~~~~~~~~~~~~~~~~~~~~~~~
> +
> +This example shows per CPU array usage.
> +
> +.. code-block:: c
> +
> +    #include <assert.h>
> +    #include <bpf/libbpf.h>
> +    #include <bpf/bpf.h>
> +
> +    int main(int argc, char **argv)
> +    {
> +	    int ncpus = libbpf_num_possible_cpus();
> +	    if (ncpus < 0)
> +		    return ncpus;
> +
> +	    int fd;
> +	    int ret = 0;
> +	    __u32 i, j;
> +	    __u32 index = 42;
> +	    long v[ncpus], value[ncpus];
> +
> +	    fd = bpf_map_create(BPF_MAP_TYPE_PERCPU_ARRAY, "example_percpu",
> +				sizeof(__u32), sizeof(long), 256, 0);
> +	    if (fd < 0)
> +		    return -1;
> +
> +	    /* fill the map with values from 0-255 for each cpu */
> +	    for (i = 0; i < 256 ; i++) {
> +		    for (j = 0; j < ncpus; j++)
> +			    v[j] = i;
> +		    ret = bpf_map_update_elem(fd, &i, &v, BPF_ANY);
> +		    if (ret < 0)
> +			    return ret;
> +	    }
> +
> +	    ret = bpf_map_lookup_elem(fd, &index, &value);
> +	    if (ret < 0)
> +		    return ret;
> +
> +	    for (j = 0; j < ncpus; j++)
> +		    assert(value[j] == 42);
> +
> +	    return ret;
> +    }
> +

What is the purpose of above snippet? Give more detailed explanation.

Thanks.

-- 
An old man doll... just what I always wanted! - Clara