netdev - Re: [PATCH bpf-next v5 02/13] xsk: Add TX timestamp and TX checksum offload support

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Date: Mon, 13 Nov 2023 14:16:49 +0100
From: Jesper Dangaard Brouer <hawk@...nel.org>
To: Stanislav Fomichev <sdf@...gle.com>, bpf@...r.kernel.org
Cc: hawk@...nel.org, ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
 martin.lau@...ux.dev, song@...nel.org, yhs@...com, john.fastabend@...il.com,
 kpsingh@...nel.org, haoluo@...gle.com, jolsa@...nel.org, kuba@...nel.org,
 toke@...nel.org, willemb@...gle.com, dsahern@...nel.org,
 magnus.karlsson@...el.com, bjorn@...nel.org, maciej.fijalkowski@...el.com,
 yoong.siang.song@...el.com, netdev@...r.kernel.org, xdp-hints@...-project.net
Subject: Re: [PATCH bpf-next v5 02/13] xsk: Add TX timestamp and TX checksum
 offload support



On 11/2/23 23:58, Stanislav Fomichev wrote:
> diff --git a/include/uapi/linux/if_xdp.h b/include/uapi/linux/if_xdp.h
> index 2ecf79282c26..b0ee7ad19b51 100644
> --- a/include/uapi/linux/if_xdp.h
> +++ b/include/uapi/linux/if_xdp.h
> @@ -106,6 +106,41 @@ struct xdp_options {
>   #define XSK_UNALIGNED_BUF_ADDR_MASK \
>   	((1ULL << XSK_UNALIGNED_BUF_OFFSET_SHIFT) - 1)
>   
> +/* Request transmit timestamp. Upon completion, put it into tx_timestamp
> + * field of struct xsk_tx_metadata.
> + */
> +#define XDP_TXMD_FLAGS_TIMESTAMP		(1 << 0)
> +
> +/* Request transmit checksum offload. Checksum start position and offset
> + * are communicated via csum_start and csum_offset fields of struct
> + * xsk_tx_metadata.
> + */
> +#define XDP_TXMD_FLAGS_CHECKSUM			(1 << 1)
> +
> +/* AF_XDP offloads request. 'request' union member is consumed by the driver
> + * when the packet is being transmitted. 'completion' union member is
> + * filled by the driver when the transmit completion arrives.
> + */
> +struct xsk_tx_metadata {
> +	union {
> +		struct {
> +			__u32 flags;
> +
> +			/* XDP_TXMD_FLAGS_CHECKSUM */
> +
> +			/* Offset from desc->addr where checksumming should start. */
> +			__u16 csum_start;
> +			/* Offset from csum_start where checksum should be stored. */
> +			__u16 csum_offset;
> +		} request;
> +
> +		struct {
> +			/* XDP_TXMD_FLAGS_TIMESTAMP */
> +			__u64 tx_timestamp;
> +		} completion;
> +	};
> +};

This looks wrong to me. It looks like member @flags is not avail at
completion time.  At completion time, I assume we also want to know if
someone requested to get the timestamp for this packet (else we could
read garbage).

Another thing (I've raised this before): It would be really practical to
store an u64 opaque value at TX and then read it at Completion time.
One use-case is a forwarding application storing HW RX-time and
comparing this to TX completion time to deduce the time spend processing
the packet.

--Jesper