netdev - Re: [PATCH bpf-next v4 2/2] samples/bpf: Add xdp_sample

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Date:   Tue, 5 Jun 2018 15:24:34 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Toke Høiland-Jørgensen <toke@...e.dk>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH bpf-next v4 2/2] samples/bpf: Add xdp_sample_pkts
 example

On Tue, 05 Jun 2018 16:50:00 +0200, Toke Høiland-Jørgensen wrote:
> Add an example program showing how to sample packets from XDP using the
> perf event buffer. The example userspace program just prints the ethernet
> header for every packet sampled.
> 
> Signed-off-by: Toke Høiland-Jørgensen <toke@...e.dk>

> diff --git a/samples/bpf/xdp_sample_pkts_kern.c b/samples/bpf/xdp_sample_pkts_kern.c
> new file mode 100644
> index 000000000000..4560522ca015
> --- /dev/null
> +++ b/samples/bpf/xdp_sample_pkts_kern.c
> @@ -0,0 +1,62 @@
> +#include <linux/ptrace.h>
> +#include <linux/version.h>
> +#include <uapi/linux/bpf.h>
> +#include "bpf_helpers.h"
> +
> +#define SAMPLE_SIZE 64ul
> +#define MAX_CPUS 24

That may be a lil' too few for modern HW with hyper-threading on ;)  
My development machine says:

$ ncpus
28

128, maybe?

> +#define bpf_printk(fmt, ...)					\
> +({								\
> +	       char ____fmt[] = fmt;				\
> +	       bpf_trace_printk(____fmt, sizeof(____fmt),	\
> +				##__VA_ARGS__);			\
> +})
> +
> +struct bpf_map_def SEC("maps") my_map = {
> +	.type = BPF_MAP_TYPE_PERF_EVENT_ARRAY,
> +	.key_size = sizeof(int),
> +	.value_size = sizeof(u32),
> +	.max_entries = MAX_CPUS,
> +};
> +
> +SEC("xdp_sample")
> +int xdp_sample_prog(struct xdp_md *ctx)
> +{
> +	void *data_end = (void *)(long)ctx->data_end;
> +	void *data = (void *)(long)ctx->data;
> +
> +        /* Metadata will be in the perf event before the packet data. */
> +	struct S {
> +		u16 cookie;
> +		u16 pkt_len;
> +	} __attribute__((packed)) metadata;
> +
> +	if (data + SAMPLE_SIZE < data_end) {
> +		/* The XDP perf_event_output handler will use the upper 32 bits
> +		 * of the flags argument as a number of bytes to include of the
> +		 * packet payload in the event data. If the size is too big, the
> +		 * call to bpf_perf_event_output will fail and return -EFAULT.
> +		 *
> +		 * See bpf_xdp_event_output in net/core/filter.c.
> +		 *
> +		 * The BPF_F_CURRENT_CPU flag means that the event output fd
> +		 * will be indexed by the CPU number in the event map.
> +		 */
> +		u64 flags = (SAMPLE_SIZE << 32) | BPF_F_CURRENT_CPU;
> +		int ret;
> +
> +		metadata.cookie = 0xdead;
> +		metadata.pkt_len = (u16)(data_end - data);
> +
> +		ret = bpf_perf_event_output(ctx, &my_map, flags,
> +				      &metadata, sizeof(metadata));
> +		if(ret)

Please run checkpatch --strict on the samples.

> +			bpf_printk("perf_event_output failed: %d\n", ret);
> +	}
> +
> +	return XDP_PASS;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> +u32 _version SEC("version") = LINUX_VERSION_CODE;