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Date: Wed, 18 Aug 2021 11:49:27 +0200
From: Jesper Dangaard Brouer <jbrouer@...hat.com>
To: Kishen Maloor <kishen.maloor@...el.com>, bpf@...r.kernel.org,
netdev@...r.kernel.org, hawk@...nel.org, magnus.karlsson@...el.com,
Björn Töpel <bjorn@...nel.org>,
Andrii Nakryiko <andrii@...nel.org>,
Jithu Joseph <jithu.joseph@...el.com>
Cc: brouer@...hat.com
Subject: Re: [RFC bpf-next 2/5] libbpf: SO_TXTIME support in AF_XDP
On 03/08/2021 19.10, Kishen Maloor wrote:
> This change adds userspace support for SO_TXTIME in AF_XDP
> to include a specific TXTIME (aka "Launch Time")
> with XDP frames issued from userspace XDP applications.
>
> The userspace API has been expanded with two helper functons:
>
> - int xsk_socket__enable_so_txtime(struct xsk_socket *xsk, bool enable)
> Sets the SO_TXTIME option on the AF_XDP socket (using setsockopt()).
>
> - void xsk_umem__set_md_txtime(void *umem_area, __u64 chunkAddr,
> __s64 txtime)
> Packages the application supplied TXTIME into struct xdp_user_tx_metadata:
> struct xdp_user_tx_metadata {
Struct name is important and becomes UAPI. I'm not 100% convinced this
is a good name.
For BPF programs libbpf can at load-time lookup the 'btf_id' via:
btf_id = bpf_core_type_id_kernel(struct xdp_user_tx_metadata);
Example see[1]
[1] https://github.com/xdp-project/bpf-examples/commit/2390b4b11079
I know this is AF_XDP userspace, but I hope Andrii can help guide us
howto expose the bpf_core_type_id_kernel() API via libbpf, to be used by
the AF_XDP userspace program.
> __u64 timestamp;
> __u32 md_valid;
> __u32 btf_id;
> };
I assume this struct is intended to be BTF "described".
Struct member *names* are very important for BTF. (E.g. see how
'spinlock' have special meaning and is matched internally by kernel).
The member name 'timestamp' seems too generic. This is a very specific
'LaunchTime' feature, which could be reflected in the name.
Later it looks like you are encoding the "type" in md_valid, which I
guess it is needed as timestamps can have different "types".
E.g. some of the clockid_t types from clock_gettime(2):
CLOCK_REALTIME
CLOCK_TAI
CLOCK_MONOTONIC
CLOCK_BOOTTIME
Which of these timestamp does XDP_METADATA_USER_TX_TIMESTAMP represent?
Or what timestamp type is the expected one?
In principle we could name the member 'Launch_Time_CLOCK_TAI' to encoded
the clockid_t type in the name, but I think that would be too much (and
require too advanced BTF helpers to extract type, having a clock_type
member is easier to understand/consume from C).
> and stores it in the XDP metadata area, which precedes the XDP frame.
>
> Signed-off-by: Kishen Maloor <kishen.maloor@...el.com>
> ---
> tools/include/uapi/linux/if_xdp.h | 2 ++
> tools/include/uapi/linux/xdp_md_std.h | 14 ++++++++++++++
> tools/lib/bpf/xsk.h | 27 ++++++++++++++++++++++++++-
> 3 files changed, 42 insertions(+), 1 deletion(-)
> create mode 100644 tools/include/uapi/linux/xdp_md_std.h
>
> diff --git a/tools/include/uapi/linux/if_xdp.h b/tools/include/uapi/linux/if_xdp.h
> index a78a8096f4ce..31f81f82ed86 100644
> --- a/tools/include/uapi/linux/if_xdp.h
> +++ b/tools/include/uapi/linux/if_xdp.h
> @@ -106,6 +106,8 @@ struct xdp_desc {
> __u32 options;
> };
>
> +#define XDP_DESC_OPTION_METADATA (1 << 0)
> +
> /* UMEM descriptor is __u64 */
>
> #endif /* _LINUX_IF_XDP_H */
> diff --git a/tools/include/uapi/linux/xdp_md_std.h b/tools/include/uapi/linux/xdp_md_std.h
> new file mode 100644
> index 000000000000..f00996a61639
> --- /dev/null
> +++ b/tools/include/uapi/linux/xdp_md_std.h
> @@ -0,0 +1,14 @@
> +#ifndef _UAPI_LINUX_XDP_MD_STD_H
> +#define _UAPI_LINUX_XDP_MD_STD_H
> +
> +#include <linux/types.h>
> +
> +#define XDP_METADATA_USER_TX_TIMESTAMP 0x1
> +
> +struct xdp_user_tx_metadata {
> + __u64 timestamp;
> + __u32 md_valid;
> + __u32 btf_id;
> +};
> +
> +#endif /* _UAPI_LINUX_XDP_MD_STD_H */
> diff --git a/tools/lib/bpf/xsk.h b/tools/lib/bpf/xsk.h
> index 01c12dca9c10..1b52ffe1c9a3 100644
> --- a/tools/lib/bpf/xsk.h
> +++ b/tools/lib/bpf/xsk.h
> @@ -16,7 +16,8 @@
> #include <stdint.h>
> #include <stdbool.h>
> #include <linux/if_xdp.h>
> -
> +#include <linux/xdp_md_std.h>
> +#include <errno.h>
> #include "libbpf.h"
>
> #ifdef __cplusplus
> @@ -248,6 +249,30 @@ static inline __u64 xsk_umem__add_offset_to_addr(__u64 addr)
> LIBBPF_API int xsk_umem__fd(const struct xsk_umem *umem);
> LIBBPF_API int xsk_socket__fd(const struct xsk_socket *xsk);
>
> +/* Helpers for SO_TXTIME */
> +
> +static inline void xsk_umem__set_md_txtime(void *umem_area, __u64 addr, __s64 txtime)
> +{
> + struct xdp_user_tx_metadata *md;
> +
> + md = (struct xdp_user_tx_metadata *)&((char *)umem_area)[addr];
> +
> + md->timestamp = txtime;
> + md->md_valid |= XDP_METADATA_USER_TX_TIMESTAMP;
Is this encoding the "type" of the timestamp?
I don't see the btf_id being updated. Does that happen in another patch?
As I note above we are current;y lacking an libbpf equivalent
bpf_core_type_id_kernel() lookup function in userspace.
> +}
> +
> +static inline int xsk_socket__enable_so_txtime(struct xsk_socket *xsk, bool enable)
> +{
> + unsigned int val = (enable) ? 1 : 0;
> + int err;
> +
> + err = setsockopt(xsk_socket__fd(xsk), SOL_XDP, SO_TXTIME, &val, sizeof(val));
> +
> + if (err)
> + return -errno;
> + return 0;
> +}
> +
> #define XSK_RING_CONS__DEFAULT_NUM_DESCS 2048
> #define XSK_RING_PROD__DEFAULT_NUM_DESCS 2048
> #define XSK_UMEM__DEFAULT_FRAME_SHIFT 12 /* 4096 bytes */
>
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