Subject: Re: [PATCH net-next v4] net: Re-use and set mono_delivery_time bit for userspace tstamp packets

From: Martin KaFai Lau <[email-masked]>
Date: Thu, 14 Mar 2024 12:21:58 -0700

On 3/14/24 2:49 AM, Willem de Bruijn wrote:
>> The two bits could potentially only encode the delivery time that is allowed to
>> be forwarded without reset. 0 could mean refering back to sk_clockid and don't
>> forward. The final consumer of the forwarded skb->tstamp is the qdisc which
>> currently only has mono and tai.
> 
> So the followinng meaning of bit pair
> { skb->mono_delivery_time, skb->user_delivery_time } ?
>   
> - { 0, 0 } legacy skb->tstamp: realtime on rx
> - { 1, 0 } skb->tstamp is mono: existing behavior of mono_delivery_time bit
> - { 0, 1 } skb->tstamp is tai: analogous to mono case
> - { 1, 1 } skb->tstamp defined by skb->sk->sk_clockid

I was thinking only forward mono and tai until it is clearer how other clocks 
will be useful for forwarding between e/ingress. By resetting all skb->tstamp 
other than mono and tai, { 0, 0 } at ingress will mean realtime on rx and { 0, 0 
} at egress will mean go look skb->sk->sk_clockid.

I do like your scheme such that it is much clearer what is in skb->tstamp 
without depending on other bits like tc_at_ingress or not.

"{ 0, 1 } skb->tstamp is tai: analogous to mono case" can probably be dropped 
for now until bpf_skb_set_tstamp(BPF_SKB_TSTAMP_DELIVERY_TAI) is needed.
Otherwise, it is mostly a duplicate of "{ 1, 1 } skb->tstamp defined by 
skb->sk->sk_clockid".

The bpf_convert_tstamp_{read,write} and the helper bpf_skb_set_tstamp need to be 
changed to handle the new "user_delivery_time" bit anyway, e.g. 
bpf_skb_set_tstamp(BPF_SKB_TSTAMP_DELIVERY_MONO) needs to clear the 
"user_delivery_time" bit.

I think the "struct inet_frag_queue" also needs a new "user_delivery_time" 
field. "mono_delivery_time" is already in there.

It may as well be cleaner to combine mono_delivery_time and user_delivery_time 
into a 2 bits field like:

struct sk_buff {
	__u8 tstamp_type:2;
};

enum {
	SKB_TSTAMP_TYPE_RX_REAL = 0, /* A RX (receive) time in real */
	SKB_TSTAMP_TYPE_TX_MONO = 1, /* A TX (delivery) time in mono */

	/* A TX (delivery) time and its clock is in skb->sk->sk_clockid.
	 *
	 * BPF_SKB_TSTAMP_DELIVERY_USER should be added
	 * such that reading __sk_buff->tstamp_type will match the
	 * SKB_TSTAMP_TYPE_TX_USER.
	 *
	 * The bpf program can learn the clockid by
	 * reading skb->sk->sk_clockid.
	 *
	 * bpf_skb_set_tstamp(BPF_SKB_TSTAMP_DELIVERY_USER)
	 * should be disallowed for now until the use case
	 * is more clear. Potentially, we could allow it
	 * in the future as long as
	 * the sock_flag(sk, SOCK_TXTIME) is true at that moment.
	 */
	SKB_TSTAMP_TYPE_TX_USER = 2,

	/* UNUSED_FOR_FUTURE = 3, */
};

It will have more code churns in the first patch to rename 
s/mono_delivery_time/tstamp_type/.

wdyt?