Subject: Re: [RFC bpf-next 5/5] selftests/bpf: Test rx_timestamp metadata in xskxceiver

From: Martin KaFai Lau <[email-masked]>
Date: Thu, 27 Oct 2022 23:22:51 -0700

On 10/27/22 1:00 PM, Stanislav Fomichev wrote:
> Example on how the metadata is prepared from the BPF context
> and consumed by AF_XDP:
> 
> - bpf_xdp_metadata_have_rx_timestamp to test whether it's supported;
>    if not, I'm assuming verifier will remove this "if (0)" branch
> - bpf_xdp_metadata_rx_timestamp returns a _copy_ of metadata;
>    the program has to bpf_xdp_adjust_meta+memcpy it;
>    maybe returning a pointer is better?
> - af_xdp consumer grabs it from data-<expected_metadata_offset> and
>    makes sure timestamp is not empty
> - when loading the program, we pass BPF_F_XDP_HAS_METADATA+prog_ifindex
> 
> Cc: Martin KaFai Lau <[email-masked]>
> Cc: Jakub Kicinski <[email-masked]>
> Cc: Willem de Bruijn <[email-masked]>
> Cc: Jesper Dangaard Brouer <[email-masked]>
> Cc: Anatoly Burakov <[email-masked]>
> Cc: Alexander Lobakin <[email-masked]>
> Cc: Magnus Karlsson <[email-masked]>
> Cc: Maryam Tahhan <[email-masked]>
> Cc: [email-masked]
> Cc: [email-masked]
> Signed-off-by: Stanislav Fomichev <[email-masked]>
> ---
>   .../testing/selftests/bpf/progs/xskxceiver.c  | 22 ++++++++++++++++++
>   tools/testing/selftests/bpf/xskxceiver.c      | 23 ++++++++++++++++++-
>   2 files changed, 44 insertions(+), 1 deletion(-)
> 
> diff --git a/tools/testing/selftests/bpf/progs/xskxceiver.c b/tools/testing/selftests/bpf/progs/xskxceiver.c
> index b135daddad3a..83c879aa3581 100644
> --- a/tools/testing/selftests/bpf/progs/xskxceiver.c
> +++ b/tools/testing/selftests/bpf/progs/xskxceiver.c
> @@ -12,9 +12,31 @@ struct {
>   	__type(value, __u32);
>   } xsk SEC(".maps");
>   
> +extern int bpf_xdp_metadata_have_rx_timestamp(struct xdp_md *ctx) __ksym;
> +extern __u32 bpf_xdp_metadata_rx_timestamp(struct xdp_md *ctx) __ksym;
> +
>   SEC("xdp")
>   int rx(struct xdp_md *ctx)
>   {
> +	void *data, *data_meta;
> +	__u32 rx_timestamp;
> +	int ret;
> +
> +	if (bpf_xdp_metadata_have_rx_timestamp(ctx)) {
> +		ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(__u32));
> +		if (ret != 0)
> +			return XDP_DROP;
> +
> +		data = (void *)(long)ctx->data;
> +		data_meta = (void *)(long)ctx->data_meta;
> +
> +		if (data_meta + sizeof(__u32) > data)
> +			return XDP_DROP;
> +
> +		rx_timestamp = bpf_xdp_metadata_rx_timestamp(ctx);
> +		__builtin_memcpy(data_meta, &rx_timestamp, sizeof(__u32));
> +	}

Thanks for the patches.  I took a quick look at patch 1 and 2 but haven't had a 
chance to look at the implementation details (eg. KF_UNROLL...etc), yet.

Overall (with the example here) looks promising.  There is a lot of flexibility 
on whether the xdp prog needs any hint at all, which hint it needs, and how to 
store it.

> +
>   	return bpf_redirect_map(&xsk, ctx->rx_queue_index, XDP_PASS);
>   }
>   
> diff --git a/tools/testing/selftests/bpf/xskxceiver.c b/tools/testing/selftests/bpf/xskxceiver.c
> index 066bd691db13..ce82c89a432e 100644
> --- a/tools/testing/selftests/bpf/xskxceiver.c
> +++ b/tools/testing/selftests/bpf/xskxceiver.c
> @@ -871,7 +871,9 @@ static bool is_offset_correct(struct xsk_umem_info *umem, struct pkt_stream *pkt
>   static bool is_pkt_valid(struct pkt *pkt, void *buffer, u64 addr, u32 len)
>   {
>   	void *data = xsk_umem__get_data(buffer, addr);
> +	void *data_meta = data - sizeof(__u32);
>   	struct iphdr *iphdr = (struct iphdr *)(data + sizeof(struct ethhdr));
> +	__u32 rx_timestamp = 0;
>   
>   	if (!pkt) {
>   		ksft_print_msg("[%s] too many packets received\n", __func__);
> @@ -907,6 +909,13 @@ static bool is_pkt_valid(struct pkt *pkt, void *buffer, u64 addr, u32 len)
>   		return false;
>   	}
>   
> +	memcpy(&rx_timestamp, data_meta, sizeof(rx_timestamp));
> +	if (rx_timestamp == 0) {
> +		ksft_print_msg("Invalid metadata received: ");
> +		ksft_print_msg("got %08x, expected != 0\n", rx_timestamp);
> +		return false;
> +	}
> +
>   	return true;
>   }