Re: [RFC PATCH bpf-next v1 3/3] net: Add additional bit to support userspace timestamp type

[Martin KaFai Lau <martin.lau@linux.dev>]

On 4/10/24 1:25 PM, Abhishek Chauhan (ABC) wrote:
>>> @@ -830,6 +833,9 @@ enum skb_tstamp_type {
>>>    *		delivery_time in mono clock base (i.e. EDT).  Otherwise, the
>>>    *		skb->tstamp has the (rcv) timestamp at ingress and
>>>    *		delivery_time at egress.
>>> + *		delivery_time in mono clock base (i.e., EDT) or a clock base chosen
>>> + *		by SO_TXTIME. If zero, skb->tstamp has the (rcv) timestamp at
>>> + *		ingress.
>>>    *	@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 +966,7 @@ struct sk_buff {
>>>   	/* private: */
>>>   	__u8			__mono_tc_offset[0];
>>>   	/* public: */
>>> -	__u8			tstamp_type:1;	/* See SKB_MONO_DELIVERY_TIME_MASK */
>>> +	__u8			tstamp_type:2;	/* See SKB_MONO_DELIVERY_TIME_MASK */
>>>   #ifdef CONFIG_NET_XGRESS
>>>   	__u8			tc_at_ingress:1;	/* See TC_AT_INGRESS_MASK */

The above "tstamp_type:2" change shifted the tc_at_ingress bit.
TC_AT_INGRESS_MASK needs to be adjusted.

>>>   	__u8			tc_skip_classify:1;
>>
>> With pahole, does this have an effect on sk_buff layout?
>>
> I think it does and it also impacts BPF testing. Hence in my cover letter i have mentioned that these
> changes will impact BPF. My level of expertise is very limited to BPF hence the reason for RFC.
> That being said i am actually trying to understand/learn BPF instructions to know things better.
> I think we need to also change the offset SKB_MONO_DELIVERY_TIME_MASK and TC_AT_INGRESS_MASK
> 
> 
> #ifdef __BIG_ENDIAN_BITFIELD
> #define SKB_MONO_DELIVERY_TIME_MASK	(1 << 7) //Suspecting changes here too
> #define TC_AT_INGRESS_MASK		(1 << 6) // and here
> #else
> #define SKB_MONO_DELIVERY_TIME_MASK	(1 << 0)
> #define TC_AT_INGRESS_MASK		(1 << 1) (this might have to change to 1<<2 )

This should be (1 << 2) now. Similar adjustment for the big endian.

> #endif
> #define SKB_BF_MONO_TC_OFFSET		offsetof(struct sk_buff, __mono_tc_offset)
> 
> Also i suspect i change in /selftests/bpf/prog_tests/ctx_rewrite.c

ctx_rewrite.c tests the bpf ctx rewrite code. In this particular case, it tests
the bpf_convert_tstamp_read() and bpf_convert_tstamp_write() generate the
correct bpf instructions.
e.g. "w11 &= 3;" is testing the following in bpf_convert_tstamp_read():
		*insn++ = BPF_ALU32_IMM(BPF_AND, tmp_reg,
	 				TC_AT_INGRESS_MASK | SKB_MONO_DELIVERY_TIME_MASK);

The existing "TC_AT_INGRESS_MASK | SKB_MONO_DELIVERY_TIME_MASK" is 0x3
and it should become 0x5 if my hand counts correctly.

The patch set cannot be applied to the bpf-next:
https://patchwork.kernel.org/project/netdevbpf/patch/20240409210547.3815806-4-quic_abchauha@quicinc.com/
, so bpf CI cannot run to reproduce the issue.