netdev - Re: [PATCH net-next v4] net: Re-use and set mono_delivery

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Message-ID: <d10254cc-a908-4d81-98d2-2eed715e521f@quicinc.com>
Date: Thu, 14 Mar 2024 15:29:04 -0700
From: "Abhishek Chauhan (ABC)" <quic_abchauha@...cinc.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>
CC: Willem de Bruijn <willemdebruijn.kernel@...il.com>, <kernel@...cinc.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>, bpf
	<bpf@...r.kernel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "Alexei
 Starovoitov" <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>
Subject: Re: [PATCH net-next v4] net: Re-use and set mono_delivery_time bit
 for userspace tstamp packets



On 3/14/2024 2:48 PM, Martin KaFai Lau wrote:
> On 3/14/24 1:53 PM, Abhishek Chauhan (ABC) wrote:
>>>> The bpf_convert_tstamp_{read,write} and the helper bpf_skb_set_tstamp need to be
>>>> changed to handle the new "user_delivery_time" bit anyway, e.g.
>>>> bpf_skb_set_tstamp(BPF_SKB_TSTAMP_DELIVERY_MONO) needs to clear the
>>>> "user_delivery_time" bit.
>>>>
>>>> I think the "struct inet_frag_queue" also needs a new "user_delivery_time"
>>>> field. "mono_delivery_time" is already in there.
> 
> [ ... ]
> 
> I would think the first step is to revert this patch. I don't think much of the current patch can be reused.
> 
>> 1. I will raise one patch to introduce rename mono_delivery_time to
>> tstamp_type
> 
> Right, I expect something like this:
> 
> struct sk_buff {
>         /* ... */
> -            __u8                    mono_delivery_time:1;
> +        __u8            tstamp_type:1;
>         /* ... */
> };
> 

Okay ,This should be straight-forward. 

>> 2. I will introduce setting of userspace timestamp type as the second bit
>> whem transmit_time is set.
> 
> I expect the second patch should be introducing the enum first
> 
> enum skb_tstamp_type {
>     SKB_TSTAMP_TYPE_RX_REAL = 0, /* A RX (receive) time in real */
>     SKB_TSTAMP_TYPE_TX_MONO = 1, /* A TX (delivery) time in mono */
> };
> 
> and start doing "skb->tstamp_type = SKB_TSTAMP_TYPE_TX_MONO;" instead of
> "skb->tstamp_type = 1;"
> 
> and the same for "skb->tstamp_type = SKB_TSTAMP_TYPE_RX_REAL;" instead of
> "skb->tstamp_type = 0;"
> 
> 
> This one I am not sure but probably need to change the skb_set_delivery_time() function signature also:
> 
> static inline void skb_set_delivery_time(struct sk_buff *skb, ktime_t kt,
> -                                        bool mono)
> +                     enum skb_tstamp_type tstamp_type)
> 
This should be straight-forward as well 

> The third patch is to change tstamp_type from 1 bit to 2 bits and add SKB_TSTAMP_TYPE_TX_USER.
> 
> struct sk_buff {
>         /* ... */
> -        __u8            tstamp_type:1;
> +        __u8            tstamp_type:2;
>         /* ... */
> };
> 
> enum skb_tstamp_type {
>     SKB_TSTAMP_TYPE_RX_REAL = 0,    /* A RX (receive) time in real */
>     SKB_TSTAMP_TYPE_TX_MONO = 1,    /* A TX (delivery) time in mono */
> +    SKB_TSTAMP_TYPE_TX_USER = 2,    /* A TX (delivery) time and its clock
>                      * is in skb->sk->sk_clockid.
>                      */
>                
> };
> 
> This will shift a bit out of the byte where tstamp_type lives. It should be the "inner_protocol_type" bit by my hand count. Please check if it is directly used in bpf instruction (filter.c). As far as I look, it is not, so should be fine. Some details about bpf instruction accessible skb bit field here: https://lore.kernel.org/all/20230321014115.997841-1-kuba@kernel.org/
This is where i would need thorough reviews from you and Willem as my area of expertise is limited to part of network stack and BPF is not one of them. 
But i have plan on this and i know how to do it. 

Expect patches to be arriving to your inboxes next week, as we have a long weekend in Qualcomm 
Fingers crossed :) 

> 
> 
>> 3. This will be a first step to make the design scalable.
>> 4. Tomorrow if we have more timestamp to support, upstream community has to do is
>> update the enum and increase the bitfield from 2=>3 and so on.
>>
>> I need help from Martin to test the patch which renames the mono_delivery_time
>> to tstamp_type (Which i feel should be straight forward as the value of the bit is 1)
> 
> The bpf change is not a no-op rename of mono_delivery_time. It needs to take care of the new bit added to the tstamp_type. Please see the previous email (and I also left it in the beginning of this email).
> 
> Thus, you need to compile the selftests/bpf/ and run it to verify the changes when handling the new bit. The Documentation/bpf/bpf_devel_QA.rst has the howto details. You probably only need the newer llvm (newer gcc should work also as bpf CI has been using it) and the newer pahole. I can definitely help if there is issue in running the test_progs in selftests/bpf or you have question on making the changes in filter.c. To run the test: "./test_progs -t tc_redirect/tc_redirect_dtime"
>