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

["Abhishek Chauhan (ABC)" <quic_abchauha@quicinc.com>]

On 4/10/2024 4:25 PM, Martin KaFai Lau wrote:
> 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.
> 

so the changes will be as follows (Martin correct me if am wrong)

		//w11 is checked againt 0x5 (Binary = 101)
		N(SCHED_CLS, struct __sk_buff, tstamp),
		.read  = "r11 = *(u8 *)($ctx + sk_buff::__mono_tc_offset);"
			 "w11 &= 5;" <== here 
			 "if w11 != 0x5  goto pc+2;" <==here
			 "$dst = 0;"
			 "goto pc+1;"
			 "$dst = *(u64 *)($ctx + sk_buff::tstamp);",

		//w11 is checked againt 0x4 (100) 
		.write = "r11 = *(u8 *)($ctx + sk_buff::__mono_tc_offset);"
			 "if w11 & 0x4 goto pc+1;" <== here
			 "goto pc+2;"
			 "w11 &= -4;" <==here
			 "*(u8 *)($ctx + sk_buff::__mono_tc_offset) = r11;"
			 "*(u64 *)($ctx + sk_buff::tstamp) = $src;",


> 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.
>