Re: [PATCH net v3 2/2] selftests/bpf: cover AF_XDP multi-buffer TX software checksum

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Wed, Oct 28, 2026 at 10:50:26AM +0800, Rongzheng Wang wrote:
> Extend test_xdp_metadata to send the same UDP packet both as one TX
> descriptor and as two descriptors, and to run the existing metadata and
> checksum verification for both cases.
> 
> Reuse the existing AF_XDP setup, packet generation, completion, and receive
> helpers in xdp_metadata.c instead of adding a separate test harness.
> 
> Signed-off-by: Rongzheng Wang <testwrzwrz@gmail.com>
> ---
>  .../selftests/bpf/prog_tests/xdp_metadata.c   | 122 ++++++++++++------
>  1 file changed, 84 insertions(+), 38 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
> index 5c31054ad4a4..13de524c7fef 100644
> --- a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
> @@ -19,6 +19,8 @@
>  #define RX_NAME "veRX"
>  
>  #define UDP_PAYLOAD_BYTES 4
> +#define UDP_SPLIT_BYTES (sizeof(struct ethhdr) + sizeof(struct iphdr) + \
> +			   sizeof(struct udphdr))
>  
>  #define UDP_SOURCE_PORT 1234
>  #define AF_XDP_CONSUMER_PORT 8080
> @@ -56,13 +58,13 @@ struct xsk {
>  	struct xsk_socket *socket;
>  };
>  
> -static int open_xsk(int ifindex, struct xsk *xsk)
> +static int open_xsk(int ifindex, struct xsk *xsk, __u32 bind_flags)
>  {
>  	int mmap_flags = MAP_PRIVATE | MAP_ANONYMOUS | MAP_NORESERVE;
>  	const struct xsk_socket_config socket_config = {
>  		.rx_size = XSK_RING_PROD__DEFAULT_NUM_DESCS,
>  		.tx_size = XSK_RING_PROD__DEFAULT_NUM_DESCS,
> -		.bind_flags = XDP_COPY,
> +		.bind_flags = bind_flags,
>  	};
>  	const struct xsk_umem_config umem_config = {
>  		.fill_size = XSK_RING_PROD__DEFAULT_NUM_DESCS,
> @@ -133,31 +135,31 @@ static void close_xsk(struct xsk *xsk)
>  	munmap(xsk->umem_area, UMEM_SIZE);
>  }
>  
> -static int generate_packet(struct xsk *xsk, __u16 dst_port)
> +static int generate_packet(struct xsk *xsk, __u16 dst_port, u16 split_len)
>  {
>  	struct xsk_tx_metadata *meta;
>  	struct xdp_desc *tx_desc;
>  	struct udphdr *udph;
>  	struct ethhdr *eth;
>  	struct iphdr *iph;
> +	u16 pkt_len;
> +	u32 num, addr;
>  	void *data;
>  	__u32 idx;
> +	u8 pkt[256];

nit: breaking RCT - I know it's test but Stan payed attention to keeping
the order...

>  	int ret;
>  
> -	ret = xsk_ring_prod__reserve(&xsk->tx, 1, &idx);
> -	if (!ASSERT_EQ(ret, 1, "xsk_ring_prod__reserve"))
> +	pkt_len = sizeof(*eth) + sizeof(*iph) + sizeof(*udph) + UDP_PAYLOAD_BYTES;
> +	if (!ASSERT_LT(split_len, pkt_len, "split_len"))
>  		return -1;
>  
> -	tx_desc = xsk_ring_prod__tx_desc(&xsk->tx, idx);
> -	tx_desc->addr = idx % (UMEM_NUM / 2) * UMEM_FRAME_SIZE + sizeof(struct xsk_tx_metadata);
> -	printf("%p: tx_desc[%u]->addr=%llx\n", xsk, idx, tx_desc->addr);
> -	data = xsk_umem__get_data(xsk->umem_area, tx_desc->addr);
> -
> -	meta = data - sizeof(struct xsk_tx_metadata);
> -	memset(meta, 0, sizeof(*meta));
> -	meta->flags = XDP_TXMD_FLAGS_TIMESTAMP;
> +	num = split_len ? 2 : 1;
> +	ret = xsk_ring_prod__reserve(&xsk->tx, num, &idx);
> +	if (!ASSERT_EQ(ret, num, "xsk_ring_prod__reserve"))
> +		return -1;
>  
> -	eth = data;
> +	memset(pkt, 0, sizeof(pkt));
> +	eth = (void *)pkt;
>  	iph = (void *)(eth + 1);
>  	udph = (void *)(iph + 1);
>  
> @@ -180,18 +182,38 @@ static int generate_packet(struct xsk *xsk, __u16 dst_port)
>  	udph->source = htons(UDP_SOURCE_PORT);
>  	udph->dest = htons(dst_port);
>  	udph->len = htons(sizeof(*udph) + UDP_PAYLOAD_BYTES);
> +	memset(udph + 1, 0xAA, UDP_PAYLOAD_BYTES);
>  	udph->check = ~csum_tcpudp_magic(iph->saddr, iph->daddr,
>  					 ntohs(udph->len), IPPROTO_UDP, 0);
>  
> -	memset(udph + 1, 0xAA, UDP_PAYLOAD_BYTES);
> +	addr = idx % (UMEM_NUM / 2) * UMEM_FRAME_SIZE;
> +	tx_desc = xsk_ring_prod__tx_desc(&xsk->tx, idx);
> +	tx_desc->addr = addr + sizeof(*meta);
> +	tx_desc->len = split_len ? split_len : pkt_len;
> +	tx_desc->options = XDP_TX_METADATA;
> +	printf("%p: tx_desc[%u]->addr=%llx\n", xsk, idx, tx_desc->addr);
> +	data = xsk_umem__get_data(xsk->umem_area, tx_desc->addr);
> +	memcpy(data, pkt, tx_desc->len);
>  
> -	meta->flags |= XDP_TXMD_FLAGS_CHECKSUM;
> +	meta = data - sizeof(*meta);
> +	memset(meta, 0, sizeof(*meta));
> +	meta->flags = XDP_TXMD_FLAGS_TIMESTAMP | XDP_TXMD_FLAGS_CHECKSUM;
>  	meta->request.csum_start = sizeof(*eth) + sizeof(*iph);
>  	meta->request.csum_offset = offsetof(struct udphdr, check);
>  
> -	tx_desc->len = sizeof(*eth) + sizeof(*iph) + sizeof(*udph) + UDP_PAYLOAD_BYTES;
> -	tx_desc->options |= XDP_TX_METADATA;
> -	xsk_ring_prod__submit(&xsk->tx, 1);
> +	if (split_len) {
> +		u32 addr2 = (idx + 1) % (UMEM_NUM / 2) * UMEM_FRAME_SIZE;
> +
> +		tx_desc->options |= XDP_PKT_CONTD;
> +		tx_desc = xsk_ring_prod__tx_desc(&xsk->tx, idx + 1);
> +		tx_desc->addr = addr2 + sizeof(*meta);
> +		tx_desc->len = pkt_len - split_len;
> +		tx_desc->options = 0;
> +		data = xsk_umem__get_data(xsk->umem_area, tx_desc->addr);
> +		memcpy(data, pkt + split_len, tx_desc->len);

would it be worth exercising this up to MAX_SKB_FRAGS ?

then instead of having this flat layout you would loop over the desc
count.

> +	}
> +
> +	xsk_ring_prod__submit(&xsk->tx, num);
>  
>  	ret = sendto(xsk_socket__fd(xsk->socket), NULL, 0, MSG_DONTWAIT, NULL, 0);
>  	if (!ASSERT_GE(ret, 0, "sendto"))
> @@ -224,24 +246,26 @@ static int generate_packet_inet(void)
>  	return err;
>  }
>  
> -static void complete_tx(struct xsk *xsk)
> +static void complete_tx(struct xsk *xsk, u32 num)
>  {
>  	struct xsk_tx_metadata *meta;
>  	__u64 addr;
>  	void *data;
> -	__u32 idx;
> -
> -	if (ASSERT_EQ(xsk_ring_cons__peek(&xsk->comp, 1, &idx), 1, "xsk_ring_cons__peek")) {
> -		addr = *xsk_ring_cons__comp_addr(&xsk->comp, idx);
> -
> -		printf("%p: complete tx idx=%u addr=%llx\n", xsk, idx, addr);
> -
> -		data = xsk_umem__get_data(xsk->umem_area, addr);
> -		meta = data - sizeof(struct xsk_tx_metadata);
> -
> -		ASSERT_NEQ(meta->completion.tx_timestamp, 0, "tx_timestamp");
> -
> -		xsk_ring_cons__release(&xsk->comp, 1);
> +	__u32 idx, i;
> +
> +	if (ASSERT_EQ(xsk_ring_cons__peek(&xsk->comp, num, &idx), num,
> +		      "xsk_ring_cons__peek")) {
> +		for (i = 0; i < num; i++) {
> +			addr = *xsk_ring_cons__comp_addr(&xsk->comp, idx + i);
> +			printf("%p: complete tx idx=%u addr=%llx\n", xsk, idx + i, addr);
> +
> +			if (i == 0) {
> +				data = xsk_umem__get_data(xsk->umem_area, addr);
> +				meta = data - sizeof(*meta);
> +				ASSERT_NEQ(meta->completion.tx_timestamp, 0, "tx_timestamp");
> +			}
> +		}
> +		xsk_ring_cons__release(&xsk->comp, num);
>  	}
>  }
>  
> @@ -398,7 +422,7 @@ void test_xdp_metadata(void)
>  
>  	/* Setup separate AF_XDP for RX interface. */
>  
> -	ret = open_xsk(rx_ifindex, &rx_xsk);
> +	ret = open_xsk(rx_ifindex, &rx_xsk, XDP_COPY);
>  	if (!ASSERT_OK(ret, "open_xsk(RX_NAME)"))
>  		goto out;
>  
> @@ -462,11 +486,11 @@ void test_xdp_metadata(void)
>  
>  	/* Setup separate AF_XDP for TX interface nad send packet to the RX socket. */
>  	tx_ifindex = if_nametoindex(TX_NAME);
> -	ret = open_xsk(tx_ifindex, &tx_xsk);
> +	ret = open_xsk(tx_ifindex, &tx_xsk, XDP_COPY | XDP_USE_SG);
>  	if (!ASSERT_OK(ret, "open_xsk(TX_NAME)"))
>  		goto out;
>  
> -	if (!ASSERT_GE(generate_packet(&tx_xsk, AF_XDP_CONSUMER_PORT), 0,
> +	if (!ASSERT_GE(generate_packet(&tx_xsk, AF_XDP_CONSUMER_PORT, 0), 0,
>  		       "generate AF_XDP_CONSUMER_PORT"))
>  		goto out;
>  
> @@ -482,7 +506,29 @@ void test_xdp_metadata(void)
>  	switch_ns_to_tx(&tok);
>  	if (!ASSERT_OK_PTR(tok, "setns tx"))
>  		goto out;
> -	complete_tx(&tx_xsk);
> +	complete_tx(&tx_xsk, 1);
> +
> +	/* Exercise software checksumming of a packet spread over multiple
> +	 * TX descriptors. The checksum is only computed once the skb is
> +	 * complete, so both fragments must be accounted for.
> +	 */
> +	if (!ASSERT_GE(generate_packet(&tx_xsk, AF_XDP_CONSUMER_PORT,
> +				       UDP_SPLIT_BYTES), 0,
> +		       "generate multi-buffer AF_XDP packet"))
> +		goto out;
> +
> +	switch_ns_to_rx(&tok);
> +	if (!ASSERT_OK_PTR(tok, "setns rx"))
> +		goto out;
> +
> +	if (!ASSERT_GE(verify_xsk_metadata(&rx_xsk, true), 0,
> +		       "verify multi-buffer xsk metadata"))
> +		goto out;
> +
> +	switch_ns_to_tx(&tok);
> +	if (!ASSERT_OK_PTR(tok, "setns tx"))
> +		goto out;
> +	complete_tx(&tx_xsk, 2);
>  
>  	/* Now check metadata of packet, generated with network stack */
>  	if (!ASSERT_GE(generate_packet_inet(), 0, "generate UDP packet"))
> @@ -518,7 +564,7 @@ void test_xdp_metadata(void)
>  		goto out;
>  
>  	/* Send packet to trigger . */
> -	if (!ASSERT_GE(generate_packet(&tx_xsk, AF_XDP_CONSUMER_PORT), 0,
> +	if (!ASSERT_GE(generate_packet(&tx_xsk, AF_XDP_CONSUMER_PORT, 0), 0,
>  		       "generate freplace packet"))
>  		goto out;
>  
> -- 
> 2.39.5 (Apple Git-154)
>