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Message-ID: <f68f26dd-db46-4e08-2814-2ed3b25ddda9@nvidia.com>
Date: Mon, 23 Jan 2023 13:39:42 +0200
From: Paul Blakey <paulb@...dia.com>
To: Simon Horman <simon.horman@...igine.com>
Cc: netdev@...r.kernel.org, Saeed Mahameed <saeedm@...dia.com>,
Paolo Abeni <pabeni@...hat.com>,
Jakub Kicinski <kuba@...nel.org>,
Eric Dumazet <edumazet@...gle.com>,
Jamal Hadi Salim <jhs@...atatu.com>,
Cong Wang <xiyou.wangcong@...il.com>,
"David S. Miller" <davem@...emloft.net>,
Oz Shlomo <ozsh@...dia.com>, Jiri Pirko <jiri@...dia.com>,
Roi Dayan <roid@...dia.com>, Vlad Buslov <vladbu@...dia.com>
Subject: Re: [PATCH net-next v4 1/6] net/sched: cls_api: Support hardware miss
to tc action
On 23/01/2023 12:04, Simon Horman wrote:
> On Sun, Jan 22, 2023 at 04:55:07PM +0200, Paul Blakey wrote:
>> For drivers to support partial offload of a filter's action list,
>> add support for action miss to specify an action instance to
>> continue from in sw.
>>
>> CT action in particular can't be fully offloaded, as new connections
>> need to be handled in software. This imposes other limitations on
>> the actions that can be offloaded together with the CT action, such
>> as packet modifications.
>>
>> Assign each action on a filter's action list a unique miss_cookie
>> which drivers can then use to fill action_miss part of the tc skb
>> extension. On getting back this miss_cookie, find the action
>> instance with relevant cookie and continue classifying from there.
>>
>> Signed-off-by: Paul Blakey <paulb@...dia.com>
>> Reviewed-by: Jiri Pirko <jiri@...dia.com>
>
> ...
>
>> diff --git a/include/net/pkt_cls.h b/include/net/pkt_cls.h
>> index 4cabb32a2ad94..9ef85cf9b5328 100644
>> --- a/include/net/pkt_cls.h
>> +++ b/include/net/pkt_cls.h
>
> ...
>
>> @@ -240,21 +243,11 @@ struct tcf_exts {
>> static inline int tcf_exts_init(struct tcf_exts *exts, struct net *net,
>> int action, int police)
>> {
>> -#ifdef CONFIG_NET_CLS_ACT
>> - exts->type = 0;
>> - exts->nr_actions = 0;
>> - /* Note: we do not own yet a reference on net.
>> - * This reference might be taken later from tcf_exts_get_net().
>> - */
>> - exts->net = net;
>> - exts->actions = kcalloc(TCA_ACT_MAX_PRIO, sizeof(struct tc_action *),
>> - GFP_KERNEL);
>> - if (!exts->actions)
>> - return -ENOMEM;
>> +#ifdef CONFIG_NET_CLS
>> + return tcf_exts_init_ex(exts, net, action, police, NULL, 0, false);
>> +#else
>> + return -EOPNOTSUPP;
>> #endif
>
> nit: I think it would be nicer if there was a dummy implementation
> of tcf_exts_init_ex for the !CONFIG_NET_CLS case and #ifdefs
> could disappear from tcf_exts_init() entirely.
>
> Likewise, elsewhere in this patch there seems to be new #if/#ifdefs
> Which may be in keeping with the style of this file. But I also
> think it's a style we ought to be moving away from.
I agree, I'll do that for v5.
>
>> - exts->action = action;
>> - exts->police = police;
>> - return 0;
>> }
>>
>> /* Return false if the netns is being destroyed in cleanup_net(). Callers
>
> ...
>
>> diff --git a/net/sched/cls_api.c b/net/sched/cls_api.c
>> index 5b4a95e8a1ee0..46524ae353c5a 100644
>> --- a/net/sched/cls_api.c
>> +++ b/net/sched/cls_api.c
>
> ...
>
>> +static struct tcf_exts_miss_cookie_node *
>> +tcf_exts_miss_cookie_lookup(u64 miss_cookie, int *act_index)
>> +{
>> + union tcf_exts_miss_cookie mc = { .miss_cookie = miss_cookie, };
>> +
>> + *act_index = mc.act_index;
>> + return xa_load(&tcf_exts_miss_cookies_xa, mc.miss_cookie_base);
>> +}
>> +#endif /* IS_ENABLED(CONFIG_NET_TC_SKB_EXT) */
>> +
>> +static u64 tcf_exts_miss_cookie_get(u32 miss_cookie_base, int act_index)
>> +{
>> + union tcf_exts_miss_cookie mc = { .act_index = act_index, };
>> +
>> + if (!miss_cookie_base)
>> + return 0;
>
> nit: perhaps the compiler optimises this out, or it is otherwise
> unimportant,
> but doesn't the assignment
> of mc zero it's fields, other than .act_index only for:
>
> 1. Any assignment of mc to be unused in the !miss_cookie_base case
> 2. mc.miss_cookie_base to be reassigned, otherwise
>
It should always zero all other fields with such assignment, and both
are equivalent while we don't have any other fields.
> FWIIW, I might have gone for something more like this (*untested*).
>
> union tcf_exts_miss_cookie mc;
>
> if (!miss_cookie_base)
> return 0;
>
> mc.act_index = act_index;
> mc.miss_cookie_base = miss_cookie_base;
>
> return mc.miss_cookie;
>
>> +
>> + mc.miss_cookie_base = miss_cookie_base;
>> + return mc.miss_cookie;
>> +}
>> +
>> #ifdef CONFIG_NET_CLS_ACT
>> DEFINE_STATIC_KEY_FALSE(tc_skb_ext_tc);
>> EXPORT_SYMBOL(tc_skb_ext_tc);
>> @@ -1549,6 +1642,8 @@ static inline int __tcf_classify(struct sk_buff *skb,
>> const struct tcf_proto *orig_tp,
>> struct tcf_result *res,
>> bool compat_mode,
>> + struct tcf_exts_miss_cookie_node *n,
>> + int act_index,
>> u32 *last_executed_chain)
>> {
>> #ifdef CONFIG_NET_CLS_ACT
>> @@ -1560,13 +1655,40 @@ static inline int __tcf_classify(struct sk_buff *skb,
>> #endif
>> for (; tp; tp = rcu_dereference_bh(tp->next)) {
>> __be16 protocol = skb_protocol(skb, false);
>> - int err;
>> + int err = 0;
>>
>> - if (tp->protocol != protocol &&
>> - tp->protocol != htons(ETH_P_ALL))
>> - continue;
>> + if (n) {
>> + struct tcf_exts *exts;
>>
>> - err = tc_classify(skb, tp, res);
>> + if (n->tp_prio != tp->prio)
>> + continue;
>> +
>> + /* We re-lookup the tp and chain based on index instead
>> + * of having hard refs and locks to them, so do a sanity
>> + * check if any of tp,chain,exts was replaced by the
>> + * time we got here with a cookie from hardware.
>> + */
>> + if (unlikely(n->tp != tp || n->tp->chain != n->chain ||
>> + !tp->ops->get_exts))
>> + return TC_ACT_SHOT;
>> +
>> + exts = tp->ops->get_exts(tp, n->handle);
>
> I see that this is probably safe, but it's not entirely obvious
> that tp->ops->get_exts will never by NULL here >
> 1) It is invariant on n, which is set in a different function and is in turn
> 2) invariant on ext->act_miss being set, several patches away, in the driver.
> 3) Which is lastly invariant on being a code path relating to for
> hardware offload of a classifier where tp->ops->get_exts is defined.
>
> Or perhaps I mixed it up somehow. But I do think at a minimum this
> ought to be documented.
Yes, I check for get_exts above it, and for !exts below it.
>
> Which brings me to a more central concern with this series: it's very
> invasive and sets up a complex relationship between the core and the
> driver. Is this the right abstraction for the problem at hand?
I think so and the changes are pretty minimal, and align with what
we currently do with sw/hw cooperation.
>
>> + if (unlikely(!exts || n->exts != exts))
>> + return TC_ACT_SHOT;
>> +
>> + n = NULL;
>> +#ifdef CONFIG_NET_CLS_ACT
>> + err = tcf_action_exec(skb, exts->actions + act_index,
>> + exts->nr_actions - act_index,
>> + res);
>> +#endif
>> + } else {
>> + if (tp->protocol != protocol &&
>> + tp->protocol != htons(ETH_P_ALL))
>> + continue;
>> +
>> + err = tc_classify(skb, tp, res);
>> + }
>> #ifdef CONFIG_NET_CLS_ACT
>> if (unlikely(err == TC_ACT_RECLASSIFY && !compat_mode)) {
>> first_tp = orig_tp;
>
> ...
>
>> @@ -1606,21 +1731,33 @@ int tcf_classify(struct sk_buff *skb,
>> #if !IS_ENABLED(CONFIG_NET_TC_SKB_EXT)
>> u32 last_executed_chain = 0;
>>
>> - return __tcf_classify(skb, tp, tp, res, compat_mode,
>> + return __tcf_classify(skb, tp, tp, res, compat_mode, NULL, 0,
>> &last_executed_chain);
>> #else
>> u32 last_executed_chain = tp ? tp->chain->index : 0;
>> + struct tcf_exts_miss_cookie_node *n = NULL;
>> const struct tcf_proto *orig_tp = tp;
>> struct tc_skb_ext *ext;
>> + int act_index = 0;
>> int ret;
>>
>> if (block) {
>> ext = skb_ext_find(skb, TC_SKB_EXT);
>>
>> - if (ext && ext->chain) {
>> + if (ext && (ext->chain || ext->act_miss)) {
>> struct tcf_chain *fchain;
>> + u32 chain = ext->chain;
>
> nit: ext->chain seems to only be used once, as it was before this patch.
> Perhaps the chain local variable is an artifact of development
> and is best not added?
It is used to share the chain lookup code below without new scope or a
'?' op (e.g n ? n->chain_index : ext->chain) which I think is cleaner.
>
>> +
>> + if (ext->act_miss) {
>> + n = tcf_exts_miss_cookie_lookup(ext->act_miss_cookie,
>> + &act_index);
>> + if (!n)
>> + return TC_ACT_SHOT;
>>
>> - fchain = tcf_chain_lookup_rcu(block, ext->chain);
>> + chain = n->chain_index;
>> + }
>> +
>> + fchain = tcf_chain_lookup_rcu(block, chain);
>> if (!fchain)
>> return TC_ACT_SHOT;
>>
>
> ...
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