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Date: Mon, 22 Apr 2024 11:46:19 -0700
From: Martin KaFai Lau <martin.lau@...ux.dev>
To: "Abhishek Chauhan (ABC)" <quic_abchauha@...cinc.com>,
Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc: "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>,
Daniel Borkmann <daniel@...earbox.net>, bpf <bpf@...r.kernel.org>,
kernel@...cinc.com
Subject: Re: [RFC PATCH bpf-next v4 2/2] net: Add additional bit to support
clockid_t timestamp type
On 4/19/24 6:13 PM, Abhishek Chauhan (ABC) wrote:
>
>
> On 4/18/2024 5:30 PM, Abhishek Chauhan (ABC) wrote:
>>
>>
>> On 4/18/2024 2:57 PM, Martin KaFai Lau wrote:
>>> 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?
>>>
>> Okay i will do the same. Noted!
>>> #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.
>>> I see what you are saying.
>>>>>> +#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):
>>>
>> Let me think this through and raise it as part of the next rfc patch.
>>> 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.
>>>
> Willem and Martin,
> When do we clear the tstamp and make it 0 in bpf_convert_tstamp_read? meaning which configuration?
When the bpf prog does not check the skb->tstamp_type. It is
the "if (!prog->tstamp_type_access)" in bpf_convert_tstamp_read().
If bpf prog does not check the skb->tstamp_type and it is at ingress,
bpf prog expects recv tstamp (ie. real clock), so it needs to clear
out the tstamp (i.e read as 0 tstamp).
> I see previously(current upstream code) if mono_delivery is set and tc_ingress_mask is set
> upstream code used to set the tstamp as 0.
>
> Which means with addition of tai mask the new implementation should take care of following cases(correct me if i am wrong)
> 1. ( tai mask set + ingress mask set ) = Clear tstamp
> 2. ( mono mask set + ingress mask set ) = Clear tstamp
> 3. ( mono mask set + tai mask set + ingress mask set ) = Clear tstamp
> 4. ( No mask set ) = Clear tstamp
> 5. ( Tai mask set + mono mask set ) = Clear tstamp
No need to check the individual mono and tai bit here. Check the
tstamp_type as a whole. Like in pseudo C:
if (skb->tc_at_ingress && skb->tstamp_type)
value_reg = 0;
untested code for tstamp_read() and tstamp_write():
static struct bpf_insn *bpf_convert_tstamp_read(const struct bpf_prog *prog,
const struct bpf_insn *si,
struct bpf_insn *insn)
{
__u8 value_reg = si->dst_reg;
__u8 skb_reg = si->src_reg;
#ifdef CONFIG_NET_XGRESS
/* If the tstamp_type is read,
* the bpf prog is aware the tstamp could have delivery time.
* Thus, read skb->tstamp as is if tstamp_type_access is true.
*/
if (!prog->tstamp_type_access) {
/* AX is needed because src_reg and dst_reg could be the same */
__u8 tmp_reg = BPF_REG_AX;
*insn++ = BPF_LDX_MEM(BPF_B, tmp_reg, skb_reg, SKB_BF_MONO_TC_OFFSET);
*insn++ = BPF_JMP32_IMM(BPF_JSET, tmp_reg, TC_AT_INGRESS_MASK, 1);
/* goto <read> */
BPF_JMP_A(4);
*insn++ = BPF_JMP32_IMM(BPF_JSET, tmp_reg, SKB_TSTAMP_TYPE_MASK, 1);
/* goto <read> */
BPF_JMP_A(2);
/* skb->tc_at_ingress && skb->tstamp_type,
* read 0 as the (rcv) timestamp.
*/
*insn++ = BPF_MOV64_IMM(value_reg, 0);
*insn++ = BPF_JMP_A(1);
}
#endif
/* <read>: value_reg = skb->tstamp */
*insn++ = BPF_LDX_MEM(BPF_DW, value_reg, skb_reg,
offsetof(struct sk_buff, tstamp));
return insn;
}
static struct bpf_insn *bpf_convert_tstamp_write(const struct bpf_prog *prog,
const struct bpf_insn *si,
struct bpf_insn *insn)
{
__u8 value_reg = si->src_reg;
__u8 skb_reg = si->dst_reg;
#ifdef CONFIG_NET_XGRESS
/* If the tstamp_type is read,
* the bpf prog is aware the tstamp could have delivery time.
* Thus, write skb->tstamp as is if tstamp_type_access is true.
* Otherwise, writing at ingress will have to clear the
* mono_delivery_time (skb->tstamp_type:1)bit also.
*/
if (!prog->tstamp_type_access) {
__u8 tmp_reg = BPF_REG_AX;
*insn++ = BPF_LDX_MEM(BPF_B, tmp_reg, skb_reg, SKB_BF_MONO_TC_OFFSET);
/* Writing __sk_buff->tstamp as ingress, goto <clear> */
*insn++ = BPF_JMP32_IMM(BPF_JSET, tmp_reg, TC_AT_INGRESS_MASK, 1);
/* goto <store> */
*insn++ = BPF_JMP_A(2);
/* <clear>: skb->tstamp_type */
*insn++ = BPF_ALU32_IMM(BPF_AND, tmp_reg, ~SKB_TSTAMP_TYPE_MASK);
*insn++ = BPF_STX_MEM(BPF_B, skb_reg, tmp_reg, SKB_BF_MONO_TC_OFFSET);
}
#endif
/* <store>: skb->tstamp = tstamp */
*insn++ = BPF_RAW_INSN(BPF_CLASS(si->code) | BPF_DW | BPF_MEM,
skb_reg, value_reg, offsetof(struct sk_buff, tstamp), si->imm);
return insn;
}
>
> This leaves us with only two values which can be support which is 0x1 and 0x2
>
> This means the tstamp_type should be either 0x1(mono) and tstamp_type 0x2 (tai) to set the value_reg with tstamp
> Is my understanding correct ?
>
> Do you think the below simplified version looks okay ?
>
> static struct bpf_insn *bpf_convert_tstamp_read(const struct bpf_prog *prog,
> 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);
> #ifdef CONFIG_NET_XGRESS
> /* If the tstamp_type is read,
> * the bpf prog is aware the tstamp could have delivery time.
> * Thus, read skb->tstamp as is if tstamp_type_access is true.
> */
> if (!prog->tstamp_type_access) {
> /* AX is needed because src_reg and dst_reg could be the same */
> __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_TSTAMP_TYPE_MASK);
>
> /* If the value of tmp_reg is 7,6,5,4,3,0 which means invalid
> * configuration set the tstamp to 0, value 0x1 and 0x2
> * is correct configuration
> */
> #ifdef __BIG_ENDIAN_BITFIELD
> *insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg, 0x1 << SKB_TSTAMP_TYPE_RSH, 3);
> *insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg, 0x2 << SKB_TSTAMP_TYPE_RSH, 2);
> #endif
> *insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg, 0x1, 3);
> *insn++ = BPF_JMP32_IMM(BPF_JEQ, tmp_reg, 0x2, 2);
> #endif
> /* skb->tc_at_ingress && skb->tstamp_type:2,
> * read 0 as the (rcv) timestamp.
> */
> *insn++ = BPF_MOV64_IMM(value_reg, 0);
> *insn++ = BPF_JMP_A(1);
> }
> #endif
>
> *insn++ = BPF_LDX_MEM(BPF_DW, value_reg, skb_reg,
> offsetof(struct sk_buff, tstamp));
> return insn;
> }
>
>
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