net: sched: cls_u32 add bit to specify software only rules

In the initial implementation the only way to stop a rule from being
inserted into the hardware table was via the device feature flag.
However this doesn't work well when working on an end host system
where packets are expect to hit both the hardware and software
datapaths.

For example we can imagine a rule that will match an IP address and
increment a field. If we install this rule in both hardware and
software we may increment the field twice. To date we have only
added support for the drop action so we have been able to ignore
these cases. But as we extend the action support we will hit this
example plus more such cases. Arguably these are not even corner
cases in many working systems these cases will be common.

To avoid forcing the driver to always abort (i.e. the above example)
this patch adds a flag to add a rule in software only. A careful
user can use this flag to build software and hardware datapaths
that work together. One example we have found particularly useful
is to use hardware resources to set the skb->mark on the skb when
the match may be expensive to run in software but a mark lookup
in a hash table is cheap. The idea here is hardware can do in one
lookup what the u32 classifier may need to traverse multiple lists
and hash tables to compute. The flag is only passed down on inserts.
On deletion to avoid stale references in hardware we always try
to remove a rule if it exists.

The flags field is part of the classifier specific options. Although
it is tempting to lift this into the generic structure doing this
proves difficult do to how the tc netlink attributes are implemented
along with how the dump/change routines are called. There is also
precedence for putting seemingly generic pieces in the specific
classifier options such as TCA_U32_POLICE, TCA_U32_ACT, etc. So
although not ideal I've left FLAGS in the u32 options as well as it
simplifies the code greatly and user space has already learned how
to manage these bits ala 'tc' tool.

Another thing if trying to update a rule we require the flags to
be unchanged. This is to force user space, software u32 and
the hardware u32 to keep in sync. Thanks to Simon Horman for
catching this case.

Signed-off-by: John Fastabend <john.r.fastabend@intel.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/pkt_cls.h

@@ -392,12 +392,21 @@ struct tc_cls_u32_offload {
 	};
 };
 
-static inline bool tc_should_offload(struct net_device *dev)
+/* tca flags definitions */
+#define TCA_CLS_FLAGS_SKIP_HW 1
+
+static inline bool tc_should_offload(struct net_device *dev, u32 flags)
 {
 	if (!(dev->features & NETIF_F_HW_TC))
 		return false;
 
-	return dev->netdev_ops->ndo_setup_tc;
+	if (flags & TCA_CLS_FLAGS_SKIP_HW)
+		return false;
+
+	if (!dev->netdev_ops->ndo_setup_tc)
+		return false;
+
+	return true;
 }
 
 #endif

include/uapi/linux/pkt_cls.h

@@ -172,6 +172,7 @@ enum {
 	TCA_U32_INDEV,
 	TCA_U32_PCNT,
 	TCA_U32_MARK,
+	TCA_U32_FLAGS,
 	__TCA_U32_MAX
 };
 

net/sched/cls_u32.c

@@ -59,6 +59,7 @@ struct tc_u_knode {
 #ifdef CONFIG_CLS_U32_PERF
 	struct tc_u32_pcnt __percpu *pf;
 #endif
+	u32			flags;
 #ifdef CONFIG_CLS_U32_MARK
 	u32			val;
 	u32			mask;
@@ -434,7 +435,7 @@ static void u32_remove_hw_knode(struct tcf_proto *tp, u32 handle)
 	offload.type = TC_SETUP_CLSU32;
 	offload.cls_u32 = &u32_offload;
 
-	if (tc_should_offload(dev)) {
+	if (tc_should_offload(dev, 0)) {
 		offload.cls_u32->command = TC_CLSU32_DELETE_KNODE;
 		offload.cls_u32->knode.handle = handle;
 		dev->netdev_ops->ndo_setup_tc(dev, tp->q->handle,
@@ -442,7 +443,9 @@ static void u32_remove_hw_knode(struct tcf_proto *tp, u32 handle)
 	}
 }
 
-static void u32_replace_hw_hnode(struct tcf_proto *tp, struct tc_u_hnode *h)
+static void u32_replace_hw_hnode(struct tcf_proto *tp,
+				 struct tc_u_hnode *h,
+				 u32 flags)
 {
 	struct net_device *dev = tp->q->dev_queue->dev;
 	struct tc_cls_u32_offload u32_offload = {0};
@@ -451,7 +454,7 @@ static void u32_replace_hw_hnode(struct tcf_proto *tp, struct tc_u_hnode *h)
 	offload.type = TC_SETUP_CLSU32;
 	offload.cls_u32 = &u32_offload;
 
-	if (tc_should_offload(dev)) {
+	if (tc_should_offload(dev, flags)) {
 		offload.cls_u32->command = TC_CLSU32_NEW_HNODE;
 		offload.cls_u32->hnode.divisor = h->divisor;
 		offload.cls_u32->hnode.handle = h->handle;
@@ -471,7 +474,7 @@ static void u32_clear_hw_hnode(struct tcf_proto *tp, struct tc_u_hnode *h)
 	offload.type = TC_SETUP_CLSU32;
 	offload.cls_u32 = &u32_offload;
 
-	if (tc_should_offload(dev)) {
+	if (tc_should_offload(dev, 0)) {
 		offload.cls_u32->command = TC_CLSU32_DELETE_HNODE;
 		offload.cls_u32->hnode.divisor = h->divisor;
 		offload.cls_u32->hnode.handle = h->handle;
@@ -482,7 +485,9 @@ static void u32_clear_hw_hnode(struct tcf_proto *tp, struct tc_u_hnode *h)
 	}
 }
 
-static void u32_replace_hw_knode(struct tcf_proto *tp, struct tc_u_knode *n)
+static void u32_replace_hw_knode(struct tcf_proto *tp,
+				 struct tc_u_knode *n,
+				 u32 flags)
 {
 	struct net_device *dev = tp->q->dev_queue->dev;
 	struct tc_cls_u32_offload u32_offload = {0};
@@ -491,7 +496,7 @@ static void u32_replace_hw_knode(struct tcf_proto *tp, struct tc_u_knode *n)
 	offload.type = TC_SETUP_CLSU32;
 	offload.cls_u32 = &u32_offload;
 
-	if (tc_should_offload(dev)) {
+	if (tc_should_offload(dev, flags)) {
 		offload.cls_u32->command = TC_CLSU32_REPLACE_KNODE;
 		offload.cls_u32->knode.handle = n->handle;
 		offload.cls_u32->knode.fshift = n->fshift;
@@ -679,6 +684,7 @@ static const struct nla_policy u32_policy[TCA_U32_MAX + 1] = {
 	[TCA_U32_SEL]		= { .len = sizeof(struct tc_u32_sel) },
 	[TCA_U32_INDEV]		= { .type = NLA_STRING, .len = IFNAMSIZ },
 	[TCA_U32_MARK]		= { .len = sizeof(struct tc_u32_mark) },
+	[TCA_U32_FLAGS]		= { .type = NLA_U32 },
 };
 
 static int u32_set_parms(struct net *net, struct tcf_proto *tp,
@@ -786,6 +792,7 @@ static struct tc_u_knode *u32_init_knode(struct tcf_proto *tp,
 #endif
 	new->fshift = n->fshift;
 	new->res = n->res;
+	new->flags = n->flags;
 	RCU_INIT_POINTER(new->ht_down, n->ht_down);
 
 	/* bump reference count as long as we hold pointer to structure */
@@ -825,7 +832,7 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 	struct tc_u32_sel *s;
 	struct nlattr *opt = tca[TCA_OPTIONS];
 	struct nlattr *tb[TCA_U32_MAX + 1];
-	u32 htid;
+	u32 htid, flags = 0;
 	int err;
 #ifdef CONFIG_CLS_U32_PERF
 	size_t size;
@@ -838,6 +845,9 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 	if (err < 0)
 		return err;
 
+	if (tb[TCA_U32_FLAGS])
+		flags = nla_get_u32(tb[TCA_U32_FLAGS]);
+
 	n = (struct tc_u_knode *)*arg;
 	if (n) {
 		struct tc_u_knode *new;
@@ -845,6 +855,9 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 		if (TC_U32_KEY(n->handle) == 0)
 			return -EINVAL;
 
+		if (n->flags != flags)
+			return -EINVAL;
+
 		new = u32_init_knode(tp, n);
 		if (!new)
 			return -ENOMEM;
@@ -861,7 +874,7 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 		u32_replace_knode(tp, tp_c, new);
 		tcf_unbind_filter(tp, &n->res);
 		call_rcu(&n->rcu, u32_delete_key_rcu);
-		u32_replace_hw_knode(tp, new);
+		u32_replace_hw_knode(tp, new, flags);
 		return 0;
 	}
 
@@ -889,7 +902,7 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 		rcu_assign_pointer(tp_c->hlist, ht);
 		*arg = (unsigned long)ht;
 
-		u32_replace_hw_hnode(tp, ht);
+		u32_replace_hw_hnode(tp, ht, flags);
 		return 0;
 	}
 
@@ -940,6 +953,7 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 	RCU_INIT_POINTER(n->ht_up, ht);
 	n->handle = handle;
 	n->fshift = s->hmask ? ffs(ntohl(s->hmask)) - 1 : 0;
+	n->flags = flags;
 	tcf_exts_init(&n->exts, TCA_U32_ACT, TCA_U32_POLICE);
 	n->tp = tp;
 
@@ -972,7 +986,7 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
 
 		RCU_INIT_POINTER(n->next, pins);
 		rcu_assign_pointer(*ins, n);
-		u32_replace_hw_knode(tp, n);
+		u32_replace_hw_knode(tp, n, flags);
 		*arg = (unsigned long)n;
 		return 0;
 	}
@@ -1077,6 +1091,9 @@ static int u32_dump(struct net *net, struct tcf_proto *tp, unsigned long fh,
 		    nla_put_u32(skb, TCA_U32_LINK, ht_down->handle))
 			goto nla_put_failure;
 
+		if (n->flags && nla_put_u32(skb, TCA_U32_FLAGS, n->flags))
+			goto nla_put_failure;
+
 #ifdef CONFIG_CLS_U32_MARK
 		if ((n->val || n->mask)) {
 			struct tc_u32_mark mark = {.val = n->val,