linux-kernel - Re: [RFC PATCH bpf-next v5 2/2] net: Add additional bit to support clockid

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Message-ID: <662a6be8aed1a_1de39b2946c@willemb.c.googlers.com.notmuch>
Date: Thu, 25 Apr 2024 10:42:48 -0400
From: Willem de Bruijn <willemdebruijn.kernel@...il.com>
To: Abhishek Chauhan <quic_abchauha@...cinc.com>, 
 "David S. Miller" <davem@...emloft.net>, 
 Eric Dumazet <edumazet@...gle.com>, 
 Jakub Kicinski <kuba@...nel.org>, 
 Paolo Abeni <pabeni@...hat.com>, 
 netdev@...r.kernel.org, 
 linux-kernel@...r.kernel.org, 
 Andrew Halaney <ahalaney@...hat.com>, 
 Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
 Martin KaFai Lau <martin.lau@...nel.org>, 
 Martin KaFai Lau <martin.lau@...ux.dev>, 
 Daniel Borkmann <daniel@...earbox.net>, 
 bpf <bpf@...r.kernel.org>
Cc: kernel@...cinc.com
Subject: Re: [RFC PATCH bpf-next v5 2/2] net: Add additional bit to support
 clockid_t timestamp type

Abhishek Chauhan wrote:
> tstamp_type is now set based on actual clockid_t compressed
> into 2 bits.
> 
> To make the design scalable for future needs this commit bring in
> the change to extend the tstamp_type:1 to tstamp_type:2 to support
> other clockid_t timestamp.
> 
> We now support CLOCK_TAI as part of tstamp_type as part of this
> commit with exisiting support CLOCK_MONOTONIC and CLOCK_REALTIME.
> 
> Link: https://lore.kernel.org/netdev/bc037db4-58bb-4861-ac31-a361a93841d3@linux.dev/
> Signed-off-by: Abhishek Chauhan <quic_abchauha@...cinc.com>
> ---

> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index e464d0ebc9c1..3ad0de07d261 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -711,6 +711,8 @@ typedef unsigned char *sk_buff_data_t;
>  enum skb_tstamp_type {
>  	SKB_CLOCK_REALTIME,
>  	SKB_CLOCK_MONOTONIC,
> +	SKB_CLOCK_TAI,
> +	__SKB_CLOCK_MAX = SKB_CLOCK_TAI,
>  };
>  
>  /**
> @@ -831,8 +833,8 @@ enum skb_tstamp_type {
>   *	@decrypted: Decrypted SKB
>   *	@slow_gro: state present at GRO time, slower prepare step required
>   *	@tstamp_type: When set, skb->tstamp has the
> - *		delivery_time in mono clock base Otherwise, the
> - *		timestamp is considered real clock base.
> + *		delivery_time in mono clock base or clock base of skb->tstamp.

drop "in mono clock base or "

> + *		Otherwise, the timestamp is considered real clock base

drop this: whenever in realtime clock base, tstamp_type is zero, so
the above shorter statement always holds.

>   *	@napi_id: id of the NAPI struct this skb came from
>   *	@sender_cpu: (aka @napi_id) source CPU in XPS
>   *	@alloc_cpu: CPU which did the skb allocation.
> @@ -960,7 +962,7 @@ struct sk_buff {
>  	/* private: */
>  	__u8			__mono_tc_offset[0];
>  	/* public: */
> -	__u8			tstamp_type:1;	/* See skb_tstamp_type */
> +	__u8			tstamp_type:2;	/* See skb_tstamp_type */
>  #ifdef CONFIG_NET_XGRESS
>  	__u8			tc_at_ingress:1;	/* See TC_AT_INGRESS_MASK */
>  	__u8			tc_skip_classify:1;
> @@ -1090,15 +1092,17 @@ struct sk_buff {
>  #endif
>  #define PKT_TYPE_OFFSET		offsetof(struct sk_buff, __pkt_type_offset)
>  
> -/* if you move tc_at_ingress or mono_delivery_time
> +/* if you move tc_at_ingress or tstamp_type:2
>   * around, you also must adapt these constants.
>   */
>  #ifdef __BIG_ENDIAN_BITFIELD
> -#define SKB_MONO_DELIVERY_TIME_MASK	(1 << 7)
> -#define TC_AT_INGRESS_MASK		(1 << 6)
> +#define SKB_TSTAMP_TYPE_MASK		(3 << 6)
> +#define SKB_TSTAMP_TYPE_RSH		(6)
> +#define TC_AT_INGRESS_RSH		(5)

I had to find BPF_RSH to understand this abbreviation.

use SHIFT instead of RSH, as that is so domain specific?

> +#define TC_AT_INGRESS_MASK		(1 << 5)
>  #else
> -#define SKB_MONO_DELIVERY_TIME_MASK	(1 << 0)
> -#define TC_AT_INGRESS_MASK		(1 << 1)
> +#define SKB_TSTAMP_TYPE_MASK		(3)
> +#define TC_AT_INGRESS_MASK		(1 << 2)
>  #endif
>  #define SKB_BF_MONO_TC_OFFSET		offsetof(struct sk_buff, __mono_tc_offset)
>  

> -	if (skb->tstamp_type == BPF_SKB_TSTAMP_DELIVERY_MONO) {
> +	if (skb->tstamp_type == BPF_SKB_TSTAMP_DELIVERY_MONO ||
> +		  skb->tstamp_type == BPF_SKB_TSTAMP_DELIVERY_TAI) {

Peculiar indentation?

Just FYI that I'm not the best person to review the BPF part.
Thankfully Martin is helping you with that.