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

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Message-ID: <56499130-765d-464a-810c-45b19490986b@quicinc.com>
Date: Tue, 12 Mar 2024 21:34:58 -0700
From: "Abhishek Chauhan (ABC)" <quic_abchauha@...cinc.com>
To: Martin KaFai Lau <martin.lau@...ux.dev>,
        Willem de Bruijn
	<willemdebruijn.kernel@...il.com>
CC: <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 3/12/2024 4:52 PM, Martin KaFai Lau wrote:
> On 3/1/24 12:13 PM, Abhishek Chauhan wrote:
>> Bridge driver today has no support to forward the userspace timestamp
>> packets and ends up resetting the timestamp. ETF qdisc checks the
>> packet coming from userspace and encounters to be 0 thereby dropping
>> time sensitive packets. These changes will allow userspace timestamps
>> packets to be forwarded from the bridge to NIC drivers.
>>
>> Setting the same bit (mono_delivery_time) to avoid dropping of
>> userspace tstamp packets in the forwarding path.
>>
>> Existing functionality of mono_delivery_time remains unaltered here,
>> instead just extended with userspace tstamp support for bridge
>> forwarding path.
> 
> The patch currently broke the bpf selftest test_tc_dtime: https://github.com/kernel-patches/bpf/actions/runs/8242487344/job/22541746675
> 
> In particular, there is a uapi field __sk_buff->tstamp_type which currently has BPF_SKB_TSTAMP_DELIVERY_MONO to mean skb->tstamp has the MONO "delivery" time. BPF_SKB_TSTAMP_UNSPEC means everything else (this could be a rx timestamp at ingress or a delivery time set by user space).
> 
> __sk_buff->tstamp_type depends on skb->mono_delivery_time which does not necessarily mean mono after this patch. I thought about fixing it on the bpf side such that reading __sk_buff->tstamp_type only returns BPF_SKB_TSTAMP_DELIVERY_MONO when the skb->mono_delivery_time is set and skb->sk is IPPROTO_TCP. However, it won't work because of bpf_skb_set_tstamp().
> 
> There is a bpf helper, bpf_skb_set_tstamp(skb, tstamp, BPF_SKB_TSTAMP_DELIVERY_MONO). This helper changes both the skb->tstamp and the skb->mono_delivery_time. The expectation is this could change skb->tstamp in the ingress skb and redirect to egress sch_fq. It could also set a mono time to skb->tstamp where the udp sk->sk_clockid may not be necessary in mono and then bpf_redirect to egress sch_fq. When bpf_skb_set_tstamp(skb, tstamp, BPF_SKB_TSTAMP_DELIVERY_MONO) succeeds, reading __sk_buff->tstamp_type expects BPF_SKB_TSTAMP_DELIVERY_MONO also.
> 
> I ran out of idea to solve this uapi breakage.
> 
> I am afraid it may need to go back to v1 idea and use another bit (user_delivery_time) in the skb.
> 
I am okay to switch back to version 1 of this patch by adding another bit called userspace_time_stamp as that was the initial intent. 

Martin what do you suggest ?