[xdp-hints] Re: [PATCH RFCv2 bpf-next 04/18] net: create xdp_hints_common and set functions

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 07-Sep-22 4:45 PM, Jesper Dangaard Brouer wrote:
> XDP-hints via BTF are about giving drivers the ability to extend the
> common set of hardware offload hints in a flexible way.
> 
> This patch start out with defining the common set, based on what is
> used available in the SKB. Having this as a common struct in core
> vmlinux makes it easier to implement xdp_frame to SKB conversion
> routines as normal C-code, see later patches.
> 
> Drivers can redefine the layout of the entire metadata area, but are
> encouraged to use this common struct as the base, on which they can
> extend on top for their extra hardware offload hints. When doing so,
> drivers can mark the xdp_buff (and xdp_frame) with flags indicating
> this it compatible with the common struct.
> 
> Patch also provides XDP-hints driver helper functions for updating the
> common struct. Helpers gets inlined and are defined for maximum
> performance, which does require some extra care in drivers, e.g. to
> keep track of flags to reduce data dependencies, see code DOC.
> 
> Userspace and BPF-prog's MUST not consider the common struct UAPI.
> The common struct (and enum flags) are only exposed via BTF, which
> implies consumers must read and decode this BTF before using/consuming
> data layout.
> 
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
> ---
>   include/net/xdp.h |  147 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>   net/core/xdp.c    |    5 ++
>   2 files changed, 152 insertions(+)
> 
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 04c852c7a77f..ea5836ccee82 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -8,6 +8,151 @@
>   
>   #include <linux/skbuff.h> /* skb_shared_info */
>   
> +/**
> + * struct xdp_hints_common - Common XDP-hints offloads shared with netstack
> + * @btf_full_id: The modules BTF object + type ID for specific struct
> + * @vlan_tci: Hardware provided VLAN tag + proto type in @xdp_hints_flags
> + * @rx_hash32: Hardware provided RSS hash value
> + * @xdp_hints_flags: see &enum xdp_hints_flags
> + *
> + * This structure contains the most commonly used hardware offloads hints
> + * provided by NIC drivers and supported by the SKB.
> + *
> + * Driver are expected to extend this structure by include &struct
> + * xdp_hints_common as part of the drivers own specific xdp_hints struct's, but
> + * at the end-of their struct given XDP metadata area grows backwards.
> + *
> + * The member @btf_full_id is populated by driver modules to uniquely identify
> + * the BTF struct.  The high 32-bits store the modules BTF object ID and the
> + * lower 32-bit the BTF type ID within that BTF object.
> + */
> +struct xdp_hints_common {
> +	union {
> +		__wsum		csum;
> +		struct {
> +			__u16	csum_start;
> +			__u16	csum_offset;
> +		};
> +	};
> +	u16 rx_queue;
> +	u16 vlan_tci;
> +	u32 rx_hash32;
> +	u32 xdp_hints_flags;
> +	u64 btf_full_id; /* BTF object + type ID */
> +} __attribute__((aligned(4))) __attribute__((packed));

I'm assuming any Tx metadata will have to go before the Rx checksum union?

> +
> +
> +/**
> + * enum xdp_hints_flags - flags used by &struct xdp_hints_common
> + *
> + * The &enum xdp_hints_flags have reserved the first 16 bits for common flags
> + * and drivers can introduce use their own flags bits from BIT(16). For
> + * BPF-progs to find these flags (via BTF) drivers should define an enum
> + * xdp_hints_flags_driver.
> + */
> +enum xdp_hints_flags {
> +	HINT_FLAG_CSUM_TYPE_BIT0  = BIT(0),
> +	HINT_FLAG_CSUM_TYPE_BIT1  = BIT(1),
> +	HINT_FLAG_CSUM_TYPE_MASK  = 0x3,
> +
> +	HINT_FLAG_CSUM_LEVEL_BIT0 = BIT(2),
> +	HINT_FLAG_CSUM_LEVEL_BIT1 = BIT(3),
> +	HINT_FLAG_CSUM_LEVEL_MASK = 0xC,
> +	HINT_FLAG_CSUM_LEVEL_SHIFT = 2,
> +
> +	HINT_FLAG_RX_HASH_TYPE_BIT0 = BIT(4),
> +	HINT_FLAG_RX_HASH_TYPE_BIT1 = BIT(5),
> +	HINT_FLAG_RX_HASH_TYPE_MASK = 0x30,
> +	HINT_FLAG_RX_HASH_TYPE_SHIFT = 0x4,
> +
> +	HINT_FLAG_RX_QUEUE = BIT(7),
> +
> +	HINT_FLAG_VLAN_PRESENT            = BIT(8),
> +	HINT_FLAG_VLAN_PROTO_ETH_P_8021Q  = BIT(9),
> +	HINT_FLAG_VLAN_PROTO_ETH_P_8021AD = BIT(10),
> +	/* Flags from BIT(16) can be used by drivers */

If we assumed we also have Tx section, would 16 bits be enough? For a 
basic implementation of UDP checksumming, AF_XDP would need 3x16 more 
bits (to store L2/L3/L4 offsets) plus probably a flag field indicating 
presence of each. Is there any way to expand common fields in the future 
(or is it at all intended to be expandable)?

-- 
Thanks,
Anatoly