netdev - Re: [RFC PATCH bpf-next v4 2/2] net: Add additional bit to support clockid

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Message-ID: <cb922600-783e-4741-be85-260d1ded5bdb@quicinc.com>
Date: Thu, 18 Apr 2024 13:10:08 -0700
From: "Abhishek Chauhan (ABC)" <quic_abchauha@...cinc.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.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>,
        "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 v4 2/2] net: Add additional bit to support
 clockid_t timestamp type



On 4/18/2024 12:06 PM, Willem de Bruijn wrote:
> 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>
>>  
>>  /**
>> - * tstamp_type:1 can take 2 values each
>> + * tstamp_type:2 can take 4 values each
>>   * represented by time base in skb
>>   * 0x0 => real timestamp_type
>>   * 0x1 => mono timestamp_type
>> + * 0x2 => tai timestamp_type
>> + * 0x3 => undefined timestamp_type
> 
> Same point as previous patch about comment that repeats name.
> 
Will take care, Noted!
>> @@ -833,7 +836,8 @@ enum skb_tstamp_type {
>>   *	@tstamp_type: When set, skb->tstamp has the
>>   *		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 at egress or skb->tstamp defined by skb->sk->sk_clockid
>> + *		coming from userspace
> 
> I would simplify the comment: clock base of skb->tstamp.
> Already in the first patch.
> 
Will take care, Noted!
>>   *	@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.
>> @@ -961,7 +965,7 @@ struct sk_buff {
>>  	/* private: */
>>  	__u8			__mono_tc_offset[0];
>>  	/* public: */
>> -	__u8			tstamp_type:1;	/* See SKB_CLOCK_*_MASK */
>> +	__u8			tstamp_type:2;	/* See skb_tstamp_type enum */
> 
> Probably good to call out that according to pahole this fills a hole.
> 
I will do that . 
>>  #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?
> 
I was thinking to keep it as TAI because it will confuse developers. I hope thats okay. 
>> +#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);
>>  
>>  	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);
> 
> 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. 

>