Re: [RFC PATCH bpf-next v4 2/2] net: Add additional bit to support clockid_t timestamp type

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

On 4/18/24 1:10 PM, Abhishek Chauhan (ABC) wrote:
>>>   #ifdef CONFIG_NET_XGRESS
>>>   	__u8			tc_at_ingress:1;	/* See TC_AT_INGRESS_MASK */
>>>   	__u8			tc_skip_classify:1;
>>> @@ -1096,10 +1100,12 @@ struct sk_buff {
>>>    */
>>>   #ifdef __BIG_ENDIAN_BITFIELD
>>>   #define SKB_MONO_DELIVERY_TIME_MASK	(1 << 7)
>>> -#define TC_AT_INGRESS_MASK		(1 << 6)
>>> +#define SKB_TAI_DELIVERY_TIME_MASK	(1 << 6)
>>
>> SKB_TSTAMP_TYPE_BIT2_MASK?

nit. Shorten it to just SKB_TSTAMP_TYPE_MASK?

#ifdef __BIG_ENDIAN_BITFIELD
#define SKB_TSTAMP_TYPE_MASK	(3 << 6)
#define SKB_TSTAMP_TYPE_RSH	(6)	/* more on this later */
#else
#define SKB_TSTAMP_TYPE_MASK	(3)
#endif

>>
> I was thinking to keep it as TAI because it will confuse developers. I hope thats okay.

I think it is not very useful to distinguish each bit since it is an enum value 
now. It becomes more like the "pkt_type:3" and its PKT_TYPE_MAX.

>>> +#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_TAI_DELIVERY_TIME_MASK	(1 << 1)
>>> +#define TC_AT_INGRESS_MASK		(1 << 2)
>>>   #endif
>>>   #define SKB_BF_MONO_TC_OFFSET		offsetof(struct sk_buff, __mono_tc_offset)
>>>   
>>> @@ -4206,6 +4212,11 @@ static inline void skb_set_delivery_time(struct sk_buff *skb, ktime_t kt,
>>>   	case CLOCK_MONOTONIC:
>>>   		skb->tstamp_type = SKB_CLOCK_MONO;
>>>   		break;
>>> +	case CLOCK_TAI:
>>> +		skb->tstamp_type = SKB_CLOCK_TAI;
>>> +		break;
>>> +	default:
>>> +		WARN_ONCE(true, "clockid %d not supported", tstamp_type);
>>
>> and set to 0 and default tstamp_type?
>> Actually thinking about it. I feel if its unsupported just fall back to default is the correct thing. I will take care of this.
>>>   	}
>>>   }
>>
>>>   >
>>   @@ -9372,10 +9378,16 @@ static struct bpf_insn *bpf_convert_tstamp_type_read(const struct bpf_insn *si,
>>>   	*insn++ = BPF_LDX_MEM(BPF_B, tmp_reg, skb_reg,
>>>   			      SKB_BF_MONO_TC_OFFSET);
>>>   	*insn++ = BPF_JMP32_IMM(BPF_JSET, tmp_reg,
>>> -				SKB_MONO_DELIVERY_TIME_MASK, 2);
>>> +				SKB_MONO_DELIVERY_TIME_MASK | SKB_TAI_DELIVERY_TIME_MASK, 2);
>>> +	*insn++ = BPF_JMP32_IMM(BPF_JSET, tmp_reg,
>>> +				SKB_MONO_DELIVERY_TIME_MASK, 3);
>>> +	*insn++ = BPF_JMP32_IMM(BPF_JSET, tmp_reg,
>>> +				SKB_TAI_DELIVERY_TIME_MASK, 4);
>>>   	*insn++ = BPF_MOV32_IMM(value_reg, BPF_SKB_TSTAMP_UNSPEC);
>>>   	*insn++ = BPF_JMP_A(1);
>>>   	*insn++ = BPF_MOV32_IMM(value_reg, BPF_SKB_TSTAMP_DELIVERY_MONO);
>>> +	*insn++ = BPF_JMP_A(1);
>>> +	*insn++ = BPF_MOV32_IMM(value_reg, BPF_SKB_TSTAMP_DELIVERY_TAI);

With SKB_TSTAMP_TYPE_MASK defined like above, this could be simplified like this 
(untested):

static struct bpf_insn *bpf_convert_tstamp_type_read(const struct bpf_insn *si,
                                                      struct bpf_insn *insn)
{
	__u8 value_reg = si->dst_reg;
	__u8 skb_reg = si->src_reg;

	BUILD_BUG_ON(__SKB_CLOCK_MAX != BPF_SKB_TSTAMP_DELIVERY_TAI);
	*insn++ = BPF_LDX_MEM(BPF_B, value_reg, skb_reg, SKB_BF_MONO_TC_OFFSET);
	*insn++ = BPF_ALU32_IMM(BPF_AND, value_reg, SKB_TSTAMP_TYPE_MASK);
#ifdef __BIG_ENDIAN_BITFIELD
	*insn++ = BPF_ALU32_IMM(BPF_RSH, value_reg, SKB_TSTAMP_TYPE_RSH);
#else
	BUILD_BUG_ON(!(SKB_TSTAMP_TYPE_MASK & 0x1));
#endif

	return insn;
}

>>>   
>>>   	return insn;
>>>   }
>>> @@ -9418,10 +9430,26 @@ static struct bpf_insn *bpf_convert_tstamp_read(const struct bpf_prog *prog,
>>>   		__u8 tmp_reg = BPF_REG_AX;
>>>   
>>>   		*insn++ = BPF_LDX_MEM(BPF_B, tmp_reg, skb_reg, SKB_BF_MONO_TC_OFFSET);
>>> +		/*check if all three bits are set*/
>>>   		*insn++ = BPF_ALU32_IMM(BPF_AND, tmp_reg,
>>> -					TC_AT_INGRESS_MASK | SKB_MONO_DELIVERY_TIME_MASK);
>>> -		*insn++ = BPF_JMP32_IMM(BPF_JNE, tmp_reg,
>>> -					TC_AT_INGRESS_MASK | SKB_MONO_DELIVERY_TIME_MASK, 2);
>>> +					TC_AT_INGRESS_MASK | SKB_MONO_DELIVERY_TIME_MASK |
>>> +					SKB_TAI_DELIVERY_TIME_MASK);
>>> +		/*if all 3 bits are set jump 3 instructions and clear the register */
>>> +		*insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg,
>>> +					TC_AT_INGRESS_MASK | SKB_MONO_DELIVERY_TIME_MASK |
>>> +					SKB_TAI_DELIVERY_TIME_MASK, 4);
>>> +		/*Now check Mono is set with ingress mask if so clear */
>>> +		*insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg,
>>> +					TC_AT_INGRESS_MASK | SKB_MONO_DELIVERY_TIME_MASK, 3);
>>> +		/*Now Check tai is set with ingress mask if so clear */
>>> +		*insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg,
>>> +					TC_AT_INGRESS_MASK | SKB_TAI_DELIVERY_TIME_MASK, 2);
>>> +		/*Now Check tai and mono are set if so clear */
>>> +		*insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg,
>>> +					SKB_MONO_DELIVERY_TIME_MASK |
>>> +					SKB_TAI_DELIVERY_TIME_MASK, 1);

Same as the bpf_convert_tstamp_type_read, this could be simplified with 
SKB_TSTAMP_TYPE_MASK.

>>
>> This looks as if all JEQ result in "if so clear"?
>>
>> Is the goal to only do something different for the two bits being 0x1,
>> can we have a single test with a two-bit mask, rather than four tests?
>>
> I think Martin wanted to take care of TAI as well. I will wait for his comment here
> 
> My Goal was to take care of invalid combos which does not hold valid
> 1. If all 3 bits are set => invalid combo (Test case written is Insane)
> 2. If 2 bits are set (tai+mono)(Test case written is Insane) => this cannot happen (because clock base can only be one in skb)
> 3. If 2 bit are set (ingress + tai/mono) => This is existing logic + tai being added (clear tstamp in ingress)
> 4. For all other cases go ahead and fill in the tstamp in the dest register.

If it is to ensure no new type is added without adding 
BPF_SKB_TSTAMP_DELIVERY_XYZ, I would simplify this runtime bpf insns here and 
use a BUILD_BUG_ON to catch it at compile time. Something like,

enum skb_tstamp_type {
         SKB_CLOCK_REAL, /* Time base is skb is REALTIME */
         SKB_CLOCK_MONO, /* Time base is skb is MONOTONIC */
  	SKB_CLOCK_TAI,  /* Time base in skb is TAI */
         __SKB_CLOCK_MAX = SKB_CLOCK_TAI,
};

/* Same one used in the bpf_convert_tstamp_type_read() above */
BUILD_BUG_ON(__SKB_CLOCK_MAX != BPF_SKB_TSTAMP_DELIVERY_TAI);

Another thing is, the UDP test in test_tc_dtime.c probably needs to be adjusted, 
the userspace is using the CLOCK_TAI in SO_TXTIME and it is getting forwarded now.