netdev - Re: [PATCH bpf-next v2 1/2] net/smc: Introduce BPF injection capability for SMC

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Date:   Thu, 23 Mar 2023 13:46:40 -0700
From:   Martin KaFai Lau <martin.lau@...ux.dev>
To:     "D. Wythe" <alibuda@...ux.alibaba.com>
Cc:     kuba@...nel.org, davem@...emloft.net, netdev@...r.kernel.org,
        linux-s390@...r.kernel.org, linux-rdma@...r.kernel.org,
        bpf@...r.kernel.org, kgraul@...ux.ibm.com, wenjia@...ux.ibm.com,
        jaka@...ux.ibm.com, ast@...nel.org, daniel@...earbox.net,
        andrii@...nel.org
Subject: Re: [PATCH bpf-next v2 1/2] net/smc: Introduce BPF injection
 capability for SMC

On 3/9/23 3:49 AM, D. Wythe wrote:
>>> --- /dev/null
>>> +++ b/net/smc/bpf_smc_struct_ops.c
>>> @@ -0,0 +1,146 @@
>>> +// SPDX-License-Identifier: GPL-2.0
>>> +
>>> +#include <linux/kernel.h>
>>> +#include <linux/bpf_verifier.h>
>>> +#include <linux/btf_ids.h>
>>> +#include <linux/bpf.h>
>>> +#include <linux/btf.h>
>>> +#include <net/sock.h>
>>> +#include <net/smc.h>
>>> +
>>> +extern struct bpf_struct_ops smc_sock_negotiator_ops;
>>> +
>>> +DEFINE_RWLOCK(smc_sock_negotiator_ops_rwlock);
>>> +struct smc_sock_negotiator_ops *negotiator;
>>
>> Is it sure one global negotiator (policy) will work for all smc_sock? or each 
>> sk should have its own negotiator and the negotiator is selected by setsockopt.
>>
> This is really a good question,  we can really consider adding an independent 
> negotiator for each sock.
> 
> But just like the TCP congestion control , the global negotiator can be used for 
> sock without
> 
> special requirements.

It is different from TCP congestion control (CC). TCP CC has a global default 
but each sk can select what tcp-cc to use and there can be multiple tcp-cc 
registered under different names.

It sounds like smc using tcp_sock should be able to have different negotiator 
also (eg. based on dst IP or some other tcp connection characteristic). The 
tcp-cc registration, per-sock selection and the rcu_read_lock+refcnt are well 
understood and there are other bpf infrastructure to support the per sock tcp-cc 
selection (like bpf_setsockopt).

For the network stack, there is little reason other af_* should not follow at 
the beginning considering the infrastructure has already been built. The one 
single global negotiator and reader/writer lock in this patch reads like an 
effort wanted to give it a try and see if it will be useful before implementing 
the whole thing. It is better to keep it off the tree for now until it is more 
ready.