RE: [PATCHv14 bpf-next 3/6] xdp: add a new helper for dev map multicast support

John Fastabend
2021-01-18T00:11:52Z

Hangbin Liu wrote:
> This patch is for xdp multicast support. which has been discussed
> before[0], The goal is to be able to implement an OVS-like data plane in
> XDP, i.e., a software switch that can forward XDP frames to multiple ports.
>
> To achieve this, an application needs to specify a group of interfaces
> to forward a packet to. It is also common to want to exclude one or more
> physical interfaces from the forwarding operation - e.g., to forward a
> packet to all interfaces in the multicast group except the interface it
> arrived on. While this could be done simply by adding more groups, this
> quickly leads to a combinatorial explosion in the number of groups an
> application has to maintain.
>
> To avoid the combinatorial explosion, we propose to include the ability
> to specify an "exclude group" as part of the forwarding operation. This
> needs to be a group (instead of just a single port index), because a
> physical interface can be part of a logical grouping, such as a bond
> device.
>
> Thus, the logical forwarding operation becomes a "set difference"
> operation, i.e. "forward to all ports in group A that are not also in
> group B". This series implements such an operation using device maps to
> represent the groups. This means that the XDP program specifies two
> device maps, one containing the list of netdevs to redirect to, and the
> other containing the exclude list.
>
> To achieve this, I re-implement a new helper bpf_redirect_map_multi()
> to accept two maps, the forwarding map and exclude map. The forwarding
> map could be DEVMAP or DEVMAP_HASH, but the exclude map *must* be
> DEVMAP_HASH to get better performace. If user don't want to use exclude
> map and just want simply stop redirecting back to ingress device, they
> can use flag BPF_F_EXCLUDE_INGRESS.
>
> As both bpf_xdp_redirect_map() and this new helpers are using struct
> bpf_redirect_info, I add a new ex_map and set tgt_value to NULL in the
> new helper to make a difference with bpf_xdp_redirect_map().
>
> Also I keep the general data path in net/core/filter.c, the native data
> path in kernel/bpf/devmap.c so we can use direct calls to get better
> performace.

[...]

> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index 0cf3976ce77c..0e6468cd0ab9 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -164,6 +164,7 @@ void xdp_warn(const char *msg, const char *func, const int line);
> #define XDP_WARN(msg) xdp_warn(msg, __func__, __LINE__)
>
> struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp);
> +struct xdp_frame *xdpf_clone(struct xdp_frame *xdpf);
>
> static inline
> void xdp_convert_frame_to_buff(struct xdp_frame *frame, struct xdp_buff *xdp)
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index a1ad32456f89..ecf5d117b96a 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -3830,6 +3830,27 @@ union bpf_attr {
> * Return
> * A pointer to a struct socket on success or NULL if the file is
> * not a socket.
> + *
> + * long bpf_redirect_map_multi(struct bpf_map *map, struct bpf_map *ex_map, u64 flags)
> + * Description
> + * This is a multicast implementation for XDP redirect. It will
> + * redirect the packet to ALL the interfaces in *map*, but
> + * exclude the interfaces in *ex_map*.
> + *
> + * The forwarding *map* could be either BPF_MAP_TYPE_DEVMAP or
> + * BPF_MAP_TYPE_DEVMAP_HASH. But the *ex_map* must be
> + * BPF_MAP_TYPE_DEVMAP_HASH to get better performance.

Would be good to add a note ex_map _must_ be keyed by ifindex for the
helper to work. Its the obvious way to key a hashmap, but not required
iirc.

> + *
> + * Currently the *flags* only supports *BPF_F_EXCLUDE_INGRESS*,
> + * which additionally excludes the current ingress device.
> + *
> + * See also bpf_redirect_map() as a unicast implementation,
> + * which supports redirecting packet to a specific ifindex
> + * in the map. As both helpers use struct bpf_redirect_info
> + * to store the redirect info, we will use a a NULL tgt_value
> + * to distinguish multicast and unicast redirecting.
> + * Return
> + * **XDP_REDIRECT** on success, or **XDP_ABORTED** on error.
> */

[...]

> +
> +int dev_map_enqueue_multi(struct xdp_buff *xdp, struct net_device *dev_rx,
> + struct bpf_map *map, struct bpf_map *ex_map,
> + u32 flags)
> +{
> + struct bpf_dtab_netdev *obj = NULL, *next_obj = NULL;
> + struct xdp_frame *xdpf, *nxdpf;
> + bool last_one = false;
> + int ex_ifindex;
> + u32 key, next_key;
> +
> + ex_ifindex = flags & BPF_F_EXCLUDE_INGRESS ? dev_rx->ifindex : 0;
> +
> + /* Find first available obj */
> + obj = devmap_get_next_obj(xdp, map, ex_map, NULL, &key, ex_ifindex);
> + if (!obj)
> + return 0;
> +
> + xdpf = xdp_convert_buff_to_frame(xdp);
> + if (unlikely(!xdpf))
> + return -EOVERFLOW;
> +
> + for (;;) {
> + /* Check if we still have one more available obj */
> + next_obj = devmap_get_next_obj(xdp, map, ex_map, &key,
> + &next_key, ex_ifindex);
> + if (!next_obj)
> + last_one = true;
> +
> + if (last_one) {
> + bq_enqueue(obj->dev, xdpf, dev_rx, obj->xdp_prog);
> + return 0;
> + }

Just collapse above to

if (!next_obj) {
bq_enqueue()
return
}

'last_one' is a bit pointless here.

> +
> + nxdpf = xdpf_clone(xdpf);
> + if (unlikely(!nxdpf)) {
> + xdp_return_frame_rx_napi(xdpf);
> + return -ENOMEM;
> + }
> +
> + bq_enqueue(obj->dev, nxdpf, dev_rx, obj->xdp_prog);
> +
> + /* Deal with next obj */
> + obj = next_obj;
> + key = next_key;
> + }
> +}
> +
> int dev_map_generic_redirect(struct bpf_dtab_netdev *dst, struct sk_buff *skb,
> struct bpf_prog *xdp_prog)
> {
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 3e4b5d9fce78..2139398057cf 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -4420,6 +4420,7 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
> case BPF_MAP_TYPE_DEVMAP:
> case BPF_MAP_TYPE_DEVMAP_HASH:
> if (func_id != BPF_FUNC_redirect_map &&
> + func_id != BPF_FUNC_redirect_map_multi &&
> func_id != BPF_FUNC_map_lookup_elem)
> goto error;
> break;
> @@ -4524,6 +4525,11 @@ static int check_map_func_compatibility(struct bpf_verifier_env *env,
> map->map_type != BPF_MAP_TYPE_XSKMAP)
> goto error;
> break;
> + case BPF_FUNC_redirect_map_multi:
> + if (map->map_type != BPF_MAP_TYPE_DEVMAP &&
> + map->map_type != BPF_MAP_TYPE_DEVMAP_HASH)
> + goto error;
> + break;
> case BPF_FUNC_sk_redirect_map:
> case BPF_FUNC_msg_redirect_map:
> case BPF_FUNC_sock_map_update:
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 9ab94e90d660..123efaf4ab88 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3924,12 +3924,19 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
> };
>
> static int __bpf_tx_xdp_map(struct net_device *dev_rx, void *fwd,
> - struct bpf_map *map, struct xdp_buff *xdp)
> + struct bpf_map *map, struct xdp_buff *xdp,
> + struct bpf_map *ex_map, u32 flags)
> {
> switch (map->map_type) {
> case BPF_MAP_TYPE_DEVMAP:
> case BPF_MAP_TYPE_DEVMAP_HASH:
> - return dev_map_enqueue(fwd, xdp, dev_rx);
> + /* We use a NULL fwd value to distinguish multicast
> + * and unicast forwarding
> + */
> + if (fwd)
> + return dev_map_enqueue(fwd, xdp, dev_rx);
> + else
> + return dev_map_enqueue_multi(xdp, dev_rx, map, ex_map, flags);
> case BPF_MAP_TYPE_CPUMAP:
> return cpu_map_enqueue(fwd, xdp, dev_rx);
> case BPF_MAP_TYPE_XSKMAP:
> @@ -3986,12 +3993,14 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
> {
> struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> struct bpf_map *map = READ_ONCE(ri->map);
> + struct bpf_map *ex_map = ri->ex_map;

READ_ONCE(ri->ex_map)?

> u32 index = ri->tgt_index;
> void *fwd = ri->tgt_value;
> int err;
>
> ri->tgt_index = 0;
> ri->tgt_value = NULL;
> + ri->ex_map = NULL;

WRITE_ONCE(ri->ex_map)?

> WRITE_ONCE(ri->map, NULL);

So we needed write_once, read_once pairs for ri->map do we also need them in
the ex_map case?

>
> if (unlikely(!map)) {
> @@ -4003,7 +4012,7 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>
> err = dev_xdp_enqueue(fwd, xdp, dev);
> } else {
> - err = __bpf_tx_xdp_map(dev, fwd, map, xdp);
> + err = __bpf_tx_xdp_map(dev, fwd, map, xdp, ex_map, ri->flags);
> }
>
> if (unlikely(err))
> @@ -4017,6 +4026,62 @@ int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
> }
> EXPORT_SYMBOL_GPL(xdp_do_redirect);

[...]

> +BPF_CALL_3(bpf_xdp_redirect_map_multi, struct bpf_map *, map,
> + struct bpf_map *, ex_map, u64, flags)
> +{
> + struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> +
> + /* Limit ex_map type to DEVMAP_HASH to get better performance */
> + if (unlikely((ex_map && ex_map->map_type != BPF_MAP_TYPE_DEVMAP_HASH) ||
> + flags & ~BPF_F_EXCLUDE_INGRESS))
> + return XDP_ABORTED;
> +
> + ri->tgt_index = 0;
> + /* Set the tgt_value to NULL to distinguish with bpf_xdp_redirect_map */
> + ri->tgt_value = NULL;
> + ri->flags = flags;
> + ri->ex_map = ex_map;

WRITE_ONCE?

> +
> + WRITE_ONCE(ri->map, map);
> +
> + return XDP_REDIRECT;
> +}
> +
> +static const struct bpf_func_proto bpf_xdp_redirect_map_multi_proto = {
> + .func = bpf_xdp_redirect_map_multi,
> + .gpl_only = false,
> + .ret_type = RET_INTEGER,
> + .arg1_type = ARG_CONST_MAP_PTR,
> + .arg2_type = ARG_CONST_MAP_PTR_OR_NULL,
> + .arg3_type = ARG_ANYTHING,
> +};
> +

Thanks,
John