Switches and modern SR-IOV enabled NICs may multiplex traffic from
representators and control messages over single set of hardware queues.
Control messages and muxed traffic may need ordered delivery.

Those requirements make it hard to comfortably use TC infrastructure
today unless we have a way of attaching metadata to skbs at the upper
device.  Because single set of queues is used for many netdevs stopping
TC/sched queues of all of them reliably is impossible and lower
device has to retreat to returning NETDEV_TX_BUSY and usually
has to take extra locks on the fastpath.

This patch attempts to enable port/representative devs to attach metadata
to skbs which carry port id.  This way representatives can be queueless
and all queuing can be performed at the lower netdev in the usual way.

Traffic arriving on the port/representative interfaces will be have 
metadata attached and will subsequently be queued to the lower device
for transmission.  The lower device should recognize the metadata and
translate it to HW specific format which is most likely either a special
header inserted before the network headers or descriptor/metadata fields.

Metadata is associated with the lower device by storing the netdev pointer
along with port id so that if TC decides to redirect or mirror the new 
netdev will not try to interpret it.

This is mostly for SR-IOV devices since switches don't have lower
netdevs today.

Since I don't have any real user in the tree at this point please
allow me to present a trivial example use here:

void upper_init(struct upper *upper, struct lower *lower, unsigned int id)
        upper->lower_dev = lower;

        upper->dst_meta = metadata_dst_alloc(0, METADATA_HW_PORT_MUX,
        upper->dst_meta.u.lower_dev = lower->netdev;
        upper->dst_meta.u.port_info.port_id = id;

int upper_tx(struct sk_buff *skb, struct net_device *netdev)
        struct upper *upper = netdev_priv(netdev);

        skb_dst_set_noref(skb, upper->dst_meta);

        return dev_queue_xmit(upper->lower_dev, skb);

int lower_tx(struct sk_buff *skb, struct net_device *netdev)
        struct metadata_dst *md = skb_metadata_dst(skb);
        struct lower *lower = netdev_priv(netdev);

        if (md->type == METADATA_HW_PORT_MUX &&
            md->u.lower_dev == netdev) {
                /* use md->u.port_id to set port in
                 * descriptor/metadata/do encap

Other approaches considered but found inferior:
 - in-data tags - inserting tags into data will be confusing
   to classifiers which start parsing from mac headers, also
   in-band data is less perfect and allows sufficiently privileged
   user to inject control messages from userspace (this is DSA model
   - note that in SR-IOV switchdev mode I control both upper and lower
   device which differs from DSA where lower device can be any MAC);
 - per-VFR HW queues - requiring a queue per VF is a little wasteful and
   less scalable, muxing allows us to use all PF queues to transmit
   and receive with full RSS (this is model of existing SR-IOV switchdev
   mode implementations);
 - per-VFR TC queue - we could use per-VFR queue in the lower device,
   tag traffic and TX on smaller set of HW queues but again scaling
   would suffer, we would need to lock an extra queue and we have no
   way to stop all queues when HW queues fill up reliably (this model
   would piggy back on dev_queue_xmit_accel() to select queue).

Any comments, reactions would be much appreciated!

Signed-off-by: Jakub Kicinski <jakub.kicin...@netronome.com>
 include/net/dst_metadata.h     | 35 ++++++++++++++++++++++++++++-------
 net/core/dst.c                 | 14 +++++++++-----
 net/core/filter.c              |  1 +
 net/ipv4/ip_tunnel_core.c      |  5 +++--
 net/openvswitch/flow_netlink.c |  3 ++-
 5 files changed, 43 insertions(+), 15 deletions(-)

diff --git a/include/net/dst_metadata.h b/include/net/dst_metadata.h
index 6965c8f68ade..6d7e1e4f3acd 100644
--- a/include/net/dst_metadata.h
+++ b/include/net/dst_metadata.h
@@ -5,10 +5,22 @@
 #include <net/ip_tunnels.h>
 #include <net/dst.h>
+enum metadata_type {
+struct hw_port_info {
+       struct netdevice *lower_dev;
+       u32 port_id;
 struct metadata_dst {
        struct dst_entry                dst;
+       enum metadata_type              type;
        union {
                struct ip_tunnel_info   tun_info;
+               struct hw_port_info     port_info;
        } u;
@@ -27,7 +39,7 @@ static inline struct ip_tunnel_info *skb_tunnel_info(struct 
sk_buff *skb)
        struct metadata_dst *md_dst = skb_metadata_dst(skb);
        struct dst_entry *dst;
-       if (md_dst)
+       if (md_dst && md_dst->type == METADATA_IP_TUNNEL)
                return &md_dst->u.tun_info;
        dst = skb_dst(skb);
@@ -55,7 +67,14 @@ static inline int skb_metadata_dst_cmp(const struct sk_buff 
        a = (const struct metadata_dst *) skb_dst(skb_a);
        b = (const struct metadata_dst *) skb_dst(skb_b);
-       if (!a != !b || a->u.tun_info.options_len != b->u.tun_info.options_len)
+       if (!a != !b || a->type != b->type)
+               return 1;
+       if (a->type == METADATA_HW_PORT_MUX)
+               return memcmp(&a->u.port_info, &b->u.port_info,
+                             sizeof(a->u.port_info));
+       if (a->u.tun_info.options_len != b->u.tun_info.options_len)
                return 1;
        return memcmp(&a->u.tun_info, &b->u.tun_info,
@@ -63,14 +82,16 @@ static inline int skb_metadata_dst_cmp(const struct sk_buff 
 void metadata_dst_free(struct metadata_dst *);
-struct metadata_dst *metadata_dst_alloc(u8 optslen, gfp_t flags);
-struct metadata_dst __percpu *metadata_dst_alloc_percpu(u8 optslen, gfp_t 
+struct metadata_dst *metadata_dst_alloc(u8 optslen, enum metadata_type type,
+                                       gfp_t flags);
+struct metadata_dst __percpu *
+metadata_dst_alloc_percpu(u8 optslen, enum metadata_type type, gfp_t flags);
 static inline struct metadata_dst *tun_rx_dst(int md_size)
        struct metadata_dst *tun_dst;
-       tun_dst = metadata_dst_alloc(md_size, GFP_ATOMIC);
+       tun_dst = metadata_dst_alloc(md_size, METADATA_IP_TUNNEL, GFP_ATOMIC);
        if (!tun_dst)
                return NULL;
@@ -85,11 +106,11 @@ static inline struct metadata_dst *tun_dst_unclone(struct 
sk_buff *skb)
        int md_size;
        struct metadata_dst *new_md;
-       if (!md_dst)
+       if (!md_dst || md_dst->type != METADATA_IP_TUNNEL)
                return ERR_PTR(-EINVAL);
        md_size = md_dst->u.tun_info.options_len;
-       new_md = metadata_dst_alloc(md_size, GFP_ATOMIC);
+       new_md = metadata_dst_alloc(md_size, METADATA_IP_TUNNEL, GFP_ATOMIC);
        if (!new_md)
                return ERR_PTR(-ENOMEM);
diff --git a/net/core/dst.c b/net/core/dst.c
index b5cbbe07f786..dc8c0c0b197b 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -367,7 +367,8 @@ static int dst_md_discard(struct sk_buff *skb)
        return 0;
-static void __metadata_dst_init(struct metadata_dst *md_dst, u8 optslen)
+static void __metadata_dst_init(struct metadata_dst *md_dst,
+                               enum metadata_type type, u8 optslen)
        struct dst_entry *dst;
@@ -379,9 +380,11 @@ static void __metadata_dst_init(struct metadata_dst 
*md_dst, u8 optslen)
        dst->output = dst_md_discard_out;
        memset(dst + 1, 0, sizeof(*md_dst) + optslen - sizeof(*dst));
+       md_dst->type = type;
-struct metadata_dst *metadata_dst_alloc(u8 optslen, gfp_t flags)
+struct metadata_dst *metadata_dst_alloc(u8 optslen, enum metadata_type type,
+                                       gfp_t flags)
        struct metadata_dst *md_dst;
@@ -389,7 +392,7 @@ struct metadata_dst *metadata_dst_alloc(u8 optslen, gfp_t 
        if (!md_dst)
                return NULL;
-       __metadata_dst_init(md_dst, optslen);
+       __metadata_dst_init(md_dst, type, optslen);
        return md_dst;
@@ -403,7 +406,8 @@ void metadata_dst_free(struct metadata_dst *md_dst)
-struct metadata_dst __percpu *metadata_dst_alloc_percpu(u8 optslen, gfp_t 
+struct metadata_dst __percpu *
+metadata_dst_alloc_percpu(u8 optslen, enum metadata_type type, gfp_t flags)
        int cpu;
        struct metadata_dst __percpu *md_dst;
@@ -414,7 +418,7 @@ struct metadata_dst __percpu *metadata_dst_alloc_percpu(u8 
optslen, gfp_t flags)
                return NULL;
-               __metadata_dst_init(per_cpu_ptr(md_dst, cpu), optslen);
+               __metadata_dst_init(per_cpu_ptr(md_dst, cpu), type, optslen);
        return md_dst;
diff --git a/net/core/filter.c b/net/core/filter.c
index 0920c2ac1d00..61536a7e932e 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -2386,6 +2386,7 @@ bpf_get_skb_set_tunnel_proto(enum bpf_func_id which)
                 * that is holding verifier mutex.
                md_dst = metadata_dst_alloc_percpu(IP_TUNNEL_OPTS_MAX,
+                                                  METADATA_IP_TUNNEL,
                if (!md_dst)
                        return NULL;
diff --git a/net/ipv4/ip_tunnel_core.c b/net/ipv4/ip_tunnel_core.c
index 777bc1883870..12ffbc4a4daa 100644
--- a/net/ipv4/ip_tunnel_core.c
+++ b/net/ipv4/ip_tunnel_core.c
@@ -145,10 +145,11 @@ struct metadata_dst *iptunnel_metadata_reply(struct 
metadata_dst *md,
        struct metadata_dst *res;
        struct ip_tunnel_info *dst, *src;
-       if (!md || md->u.tun_info.mode & IP_TUNNEL_INFO_TX)
+       if (!md || md->type != METADATA_IP_TUNNEL ||
+           md->u.tun_info.mode & IP_TUNNEL_INFO_TX)
                return NULL;
-       res = metadata_dst_alloc(0, flags);
+       res = metadata_dst_alloc(0, METADATA_IP_TUNNEL, flags);
        if (!res)
                return NULL;
diff --git a/net/openvswitch/flow_netlink.c b/net/openvswitch/flow_netlink.c
index ae25ded82b3b..c9971701d0af 100644
--- a/net/openvswitch/flow_netlink.c
+++ b/net/openvswitch/flow_netlink.c
@@ -2072,7 +2072,8 @@ static int validate_and_copy_set_tun(const struct nlattr 
        if (start < 0)
                return start;
-       tun_dst = metadata_dst_alloc(key.tun_opts_len, GFP_KERNEL);
+       tun_dst = metadata_dst_alloc(key.tun_opts_len, METADATA_IP_TUNNEL,
+                                    GFP_KERNEL);
        if (!tun_dst)
                return -ENOMEM;

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