Date:   Tue, 3 Jul 2018 16:01:39 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Saeed Mahameed <saeedm@....mellanox.co.il>
Cc:     brouer@...hat.com, borkmann@...earbox.net, tariqt@...lanox.com,
        alexander.h.duyck@...el.com, peter.waskiewicz.jr@...el.com,
        netdev@...r.kernel.org, john.fastabend@...il.com
Subject: Re: [RFC bpf-next 2/6] net: xdp: RX meta data infrastructure

On Tue, Jun 26, 2018 at 07:46:11PM -0700, Saeed Mahameed wrote:
> The idea from this patch is to define a well known structure for XDP meta
> data fields format and offset placement inside the xdp data meta buffer.
> 
> For that driver will need some static information to know what to
> provide and where, enters struct xdp_md_info and xdp_md_info_arr:
> 
> struct xdp_md_info {
>        __u16 present:1;
>        __u16 offset:15; /* offset from data_meta in xdp_md buffer */
> };
> 
> /* XDP meta data offsets info array
>  * present bit describes if a meta data is or will be present in xdp_md buff
>  * offset describes where a meta data is or should be placed in xdp_md buff
>  *
>  * Kernel builds this array using xdp_md_info_build helper on demand.
>  * User space builds it statically in the xdp program.
>  */
> typedef struct xdp_md_info xdp_md_info_arr[XDP_DATA_META_MAX];
> 
> Offsets in xdp_md_info_arr are always in ascending order and only for
> requested meta data:
> example : flags = XDP_FLAGS_META_HASH | XDP_FLAGS_META_VLAN;
> 
> xdp_md_info_arr mdi = {
> 	[XDP_DATA_META_HASH] = {.offset = 0, .present = 1},
> 	[XDP_DATA_META_MARK] = {.offset = 0, .present = 0},
> 	[XDP_DATA_META_VLAN] = {.offset = sizeof(struct xdp_md_hash), .present = 1},
> 	[XDP_DATA_META_CSUM] = {.offset = 0, .present = 0},
> }
> 
> i.e: hash fields will always appear first then the vlan for every
> xdp_md.
> 
> Once requested to provide xdp meta data, device driver will use a pre-built
> xdp_md_info_arr which was built via xdp_md_info_build on xdp setup,
> xdp_md_info_arr will tell the driver what is the offset of each meta data.
> 
> This patch also provides helper functions for the device drivers to store
> meta data into xdp_buff, and helper function for XDP programs to fetch
> specific xdp meta data from xdp_md buffer.
> 
> Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>
...
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 59b19b6a40d7..e320e7421565 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -2353,6 +2353,120 @@ struct xdp_md {
>  	__u32 rx_queue_index;  /* rxq->queue_index  */
>  };
>  
> +enum {
> +	XDP_DATA_META_HASH = 0,
> +	XDP_DATA_META_MARK,
> +	XDP_DATA_META_VLAN,
> +	XDP_DATA_META_CSUM_COMPLETE,
> +
> +	XDP_DATA_META_MAX,
> +};
> +
> +struct xdp_md_hash {
> +	__u32 hash;
> +	__u8  type;
> +} __attribute__((aligned(4)));
> +
> +struct xdp_md_mark {
> +	__u32 mark;
> +} __attribute__((aligned(4)));
> +
> +struct xdp_md_vlan {
> +	__u16 vlan;
> +} __attribute__((aligned(4)));
> +
> +struct xdp_md_csum {
> +	__u16 csum;
> +} __attribute__((aligned(4)));
> +
> +static const __u16 xdp_md_size[XDP_DATA_META_MAX] = {
> +	sizeof(struct xdp_md_hash), /* XDP_DATA_META_HASH */
> +	sizeof(struct xdp_md_mark), /* XDP_DATA_META_MARK */
> +	sizeof(struct xdp_md_vlan), /* XDP_DATA_META_VLAN */
> +	sizeof(struct xdp_md_csum), /* XDP_DATA_META_CSUM_COMPLETE */
> +};
> +
> +struct xdp_md_info {
> +	__u16 present:1;
> +	__u16 offset:15; /* offset from data_meta in xdp_md buffer */
> +};
> +
> +/* XDP meta data offsets info array
> + * present bit describes if a meta data is or will be present in xdp_md buff
> + * offset describes where a meta data is or should be placed in xdp_md buff
> + *
> + * Kernel builds this array using xdp_md_info_build helper on demand.
> + * User space builds it statically in the xdp program.
> + */
> +typedef struct xdp_md_info xdp_md_info_arr[XDP_DATA_META_MAX];
> +
> +static __always_inline __u8
> +xdp_data_meta_present(xdp_md_info_arr mdi, int meta_data)
> +{
> +	return mdi[meta_data].present;
> +}
> +
> +static __always_inline void
> +xdp_data_meta_set_hash(xdp_md_info_arr mdi, void *data_meta, __u32 hash, __u8 htype)
> +{
> +	struct xdp_md_hash *hash_md;
> +
> +	hash_md = (struct xdp_md_hash *)((char*)data_meta + mdi[XDP_DATA_META_HASH].offset);
> +	hash_md->hash = hash;
> +	hash_md->type = htype;
> +}
> +
> +static __always_inline struct xdp_md_hash *
> +xdp_data_meta_get_hash(xdp_md_info_arr mdi, void *data_meta)
> +{
> +	return (struct xdp_md_hash *)((char*)data_meta + mdi[XDP_DATA_META_HASH].offset);
> +}

I'm afraid this is not scalable uapi.
This looks very much mlx specific. Every NIC vendor cannot add its own
struct definitions into uapi/bpf.h. It doesn't scale.
Also doing 15 bit bitfield extraction using bpf instructions is not that simple.
mlx should consider doing plain u8 or u16 fields in firmware instead.
The metadata that "hw" provides is a joint work of actual asic and firmware.
I suspect the format can and will change with different firmware.
Baking this stuff into uapi/bpf.h is not an option.
How about we make driver+firmware provide a BTF definition of metadata that they
can provide? There can be multiple definitions of such structs.
Then in userpsace we can have BTF->plain C converter.
(bpftool practically ready to do that already).
Then the programmer can take such generated C definition, add it to .h and include
it in their programs. llvm will compile the whole thing and will include BTF
of maps, progs and this md struct in the target elf file.
During loading the kernel can check that BTF in elf is matching one-to-one
to what driver+firmware are saying they support.
No ambiguity and no possibility of mistake, since offsets and field names
are verified.
Every driver can have their own BTF for md and their own special features.
We can try to standardize the names (like vlan and csum), so xdp programs
can stay relatively portable across NICs.
Such api will address exposing asic+firmware metadata to the xdp program.
Once we tackle this problem, we'll think how to do the backward config
(to do firmware reconfig for specific BTF definition of md supplied by the prog).
What people think?

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