Subject: Re: [PATCH bpf-next 2/2] bpf, xdp: restructure redirect actions

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 19 Feb 2021 18:10:40 +0100

Björn Töpel <[email-masked]> writes:

> From: Björn Töpel <[email-masked]>
>
> The XDP_REDIRECT implementations for maps and non-maps are fairly
> similar, but obviously need to take different code paths depending on
> if the target is using a map or not. Today, the redirect targets for
> XDP either uses a map, or is based on ifindex.
>
> Here, an explicit redirect type is added to bpf_redirect_info, instead
> of the actual map. Redirect type, map item/ifindex, and the map_id (if
> any) is passed to xdp_do_redirect().
>
> In addition to making the code easier to follow, using an explicit
> type in bpf_redirect_info has a slight positive performance impact by
> avoiding a pointer indirection for the map type lookup, and instead
> use the cacheline for bpf_redirect_info.
>
> Since the actual map is not passed via bpf_redirect_info anymore, the
> map lookup is only done in the BPF helper. This means that the
> bpf_clear_redirect_map() function can be removed. The actual map item
> is RCU protected.
>
> The bpf_redirect_info flags member is not used by XDP, and not
> read/written any more. The map member is only written to when
> required/used, and not unconditionally.
>
> rfc->v1: Use map_id, and remove bpf_clear_redirect_map(). (Toke)
>
> Reviewed-by: Maciej Fijalkowski <[email-masked]>
> Signed-off-by: Björn Töpel <[email-masked]>

Very cool! Also a small nit below, but otherwise:

Acked-by: Toke Høiland-Jørgensen <[email-masked]>

> ---
>  include/linux/filter.h     |  11 ++-
>  include/trace/events/xdp.h |  66 +++++++++------
>  kernel/bpf/cpumap.c        |   1 -
>  kernel/bpf/devmap.c        |   1 -
>  net/core/filter.c          | 162 ++++++++++++++++---------------------
>  net/xdp/xskmap.c           |   1 -
>  6 files changed, 121 insertions(+), 121 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 1dedcf66b694..1f3cf2a1e116 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -646,11 +646,20 @@ struct bpf_redirect_info {
>  	u32 flags;
>  	u32 tgt_index;
>  	void *tgt_value;
> -	struct bpf_map *map;
> +	u32 map_id;
> +	u32 tgt_type;
>  	u32 kern_flags;
>  	struct bpf_nh_params nh;
>  };
>  
> +enum xdp_redirect_type {
> +	XDP_REDIR_UNSET,
> +	XDP_REDIR_DEV_IFINDEX,
> +	XDP_REDIR_DEV_MAP,
> +	XDP_REDIR_CPU_MAP,
> +	XDP_REDIR_XSK_MAP,
> +};
> +
>  DECLARE_PER_CPU(struct bpf_redirect_info, bpf_redirect_info);
>  
>  /* flags for bpf_redirect_info kern_flags */
> diff --git a/include/trace/events/xdp.h b/include/trace/events/xdp.h
> index 76a97176ab81..538321735447 100644
> --- a/include/trace/events/xdp.h
> +++ b/include/trace/events/xdp.h
> @@ -86,19 +86,15 @@ struct _bpf_dtab_netdev {
>  };
>  #endif /* __DEVMAP_OBJ_TYPE */
>  
> -#define devmap_ifindex(tgt, map)				\
> -	(((map->map_type == BPF_MAP_TYPE_DEVMAP ||	\
> -		  map->map_type == BPF_MAP_TYPE_DEVMAP_HASH)) ? \
> -	  ((struct _bpf_dtab_netdev *)tgt)->dev->ifindex : 0)
> -
>  DECLARE_EVENT_CLASS(xdp_redirect_template,
>  
>  	TP_PROTO(const struct net_device *dev,
>  		 const struct bpf_prog *xdp,
>  		 const void *tgt, int err,
> -		 const struct bpf_map *map, u32 index),
> +		 enum xdp_redirect_type type,
> +		 const struct bpf_redirect_info *ri),
>  
> -	TP_ARGS(dev, xdp, tgt, err, map, index),
> +	TP_ARGS(dev, xdp, tgt, err, type, ri),
>  
>  	TP_STRUCT__entry(
>  		__field(int, prog_id)
> @@ -111,14 +107,30 @@ DECLARE_EVENT_CLASS(xdp_redirect_template,
>  	),
>  
>  	TP_fast_assign(
> +		u32 ifindex = 0, map_id = 0, index = ri->tgt_index;
> +
> +		switch (type) {
> +		case XDP_REDIR_DEV_MAP:
> +			ifindex = ((struct _bpf_dtab_netdev *)tgt)->dev->ifindex;
> +			fallthrough;
> +		case XDP_REDIR_CPU_MAP:
> +		case XDP_REDIR_XSK_MAP:
> +			map_id = ri->map_id;
> +			break;
> +		case XDP_REDIR_DEV_IFINDEX:
> +			ifindex = (u32)(long)tgt;
> +			break;
> +		default:
> +			break;
> +		}
> +
>  		__entry->prog_id	= xdp->aux->id;
>  		__entry->act		= XDP_REDIRECT;
>  		__entry->ifindex	= dev->ifindex;
>  		__entry->err		= err;
> -		__entry->to_ifindex	= map ? devmap_ifindex(tgt, map) :
> -						index;
> -		__entry->map_id		= map ? map->id : 0;
> -		__entry->map_index	= map ? index : 0;
> +		__entry->to_ifindex	= ifindex;
> +		__entry->map_id		= map_id;
> +		__entry->map_index	= index;
>  	),
>  
>  	TP_printk("prog_id=%d action=%s ifindex=%d to_ifindex=%d err=%d"
> @@ -133,45 +145,49 @@ DEFINE_EVENT(xdp_redirect_template, xdp_redirect,
>  	TP_PROTO(const struct net_device *dev,
>  		 const struct bpf_prog *xdp,
>  		 const void *tgt, int err,
> -		 const struct bpf_map *map, u32 index),
> -	TP_ARGS(dev, xdp, tgt, err, map, index)
> +		 enum xdp_redirect_type type,
> +		 const struct bpf_redirect_info *ri),
> +	TP_ARGS(dev, xdp, tgt, err, type, ri)
>  );
>  
>  DEFINE_EVENT(xdp_redirect_template, xdp_redirect_err,
>  	TP_PROTO(const struct net_device *dev,
>  		 const struct bpf_prog *xdp,
>  		 const void *tgt, int err,
> -		 const struct bpf_map *map, u32 index),
> -	TP_ARGS(dev, xdp, tgt, err, map, index)
> +		 enum xdp_redirect_type type,
> +		 const struct bpf_redirect_info *ri),
> +	TP_ARGS(dev, xdp, tgt, err, type, ri)
>  );
>  
>  #define _trace_xdp_redirect(dev, xdp, to)				\
> -	 trace_xdp_redirect(dev, xdp, NULL, 0, NULL, to)
> +	trace_xdp_redirect(dev, xdp, NULL, 0, XDP_REDIR_DEV_IFINDEX, NULL)
>  
>  #define _trace_xdp_redirect_err(dev, xdp, to, err)			\
> -	 trace_xdp_redirect_err(dev, xdp, NULL, err, NULL, to)
> +	trace_xdp_redirect_err(dev, xdp, NULL, err, XDP_REDIR_DEV_IFINDEX, NULL)
>  
> -#define _trace_xdp_redirect_map(dev, xdp, to, map, index)		\
> -	 trace_xdp_redirect(dev, xdp, to, 0, map, index)
> +#define _trace_xdp_redirect_map(dev, xdp, to, type, ri)		\
> +	trace_xdp_redirect(dev, xdp, to, 0, type, ri)
>  
> -#define _trace_xdp_redirect_map_err(dev, xdp, to, map, index, err)	\
> -	 trace_xdp_redirect_err(dev, xdp, to, err, map, index)
> +#define _trace_xdp_redirect_map_err(dev, xdp, to, type, ri, err)	\
> +	trace_xdp_redirect_err(dev, xdp, to, err, type, ri)
>  
>  /* not used anymore, but kept around so as not to break old programs */
>  DEFINE_EVENT(xdp_redirect_template, xdp_redirect_map,
>  	TP_PROTO(const struct net_device *dev,
>  		 const struct bpf_prog *xdp,
>  		 const void *tgt, int err,
> -		 const struct bpf_map *map, u32 index),
> -	TP_ARGS(dev, xdp, tgt, err, map, index)
> +		 enum xdp_redirect_type type,
> +		 const struct bpf_redirect_info *ri),
> +	TP_ARGS(dev, xdp, tgt, err, type, ri)
>  );
>  
>  DEFINE_EVENT(xdp_redirect_template, xdp_redirect_map_err,
>  	TP_PROTO(const struct net_device *dev,
>  		 const struct bpf_prog *xdp,
>  		 const void *tgt, int err,
> -		 const struct bpf_map *map, u32 index),
> -	TP_ARGS(dev, xdp, tgt, err, map, index)
> +		 enum xdp_redirect_type type,
> +		 const struct bpf_redirect_info *ri),
> +	TP_ARGS(dev, xdp, tgt, err, type, ri)
>  );
>  
>  TRACE_EVENT(xdp_cpumap_kthread,
> diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> index a4d2cb93cd69..b7f4d22f5c8d 100644
> --- a/kernel/bpf/cpumap.c
> +++ b/kernel/bpf/cpumap.c
> @@ -543,7 +543,6 @@ static void cpu_map_free(struct bpf_map *map)
>  	 * complete.
>  	 */
>  
> -	bpf_clear_redirect_map(map);
>  	synchronize_rcu();
>  
>  	/* For cpu_map the remote CPUs can still be using the entries
> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
> index 37ac4cde9713..b5681a98020d 100644
> --- a/kernel/bpf/devmap.c
> +++ b/kernel/bpf/devmap.c
> @@ -197,7 +197,6 @@ static void dev_map_free(struct bpf_map *map)
>  	list_del_rcu(&dtab->list);
>  	spin_unlock(&dev_map_lock);
>  
> -	bpf_clear_redirect_map(map);
>  	synchronize_rcu();
>  
>  	/* Make sure prior __dev_map_entry_free() have completed. */
> diff --git a/net/core/filter.c b/net/core/filter.c
> index fd64d768e16a..56074b88d7e2 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -3919,23 +3919,6 @@ static const struct bpf_func_proto bpf_xdp_adjust_meta_proto = {
>  	.arg2_type	= ARG_ANYTHING,
>  };
>  
> -static int __bpf_tx_xdp_map(struct net_device *dev_rx, void *fwd,
> -			    struct bpf_map *map, struct xdp_buff *xdp)
> -{
> -	switch (map->map_type) {
> -	case BPF_MAP_TYPE_DEVMAP:
> -	case BPF_MAP_TYPE_DEVMAP_HASH:
> -		return dev_map_enqueue(fwd, xdp, dev_rx);
> -	case BPF_MAP_TYPE_CPUMAP:
> -		return cpu_map_enqueue(fwd, xdp, dev_rx);
> -	case BPF_MAP_TYPE_XSKMAP:
> -		return __xsk_map_redirect(fwd, xdp);
> -	default:
> -		return -EBADRQC;
> -	}
> -	return 0;
> -}
> -
>  void xdp_do_flush(void)
>  {
>  	__dev_flush();
> @@ -3944,55 +3927,45 @@ void xdp_do_flush(void)
>  }
>  EXPORT_SYMBOL_GPL(xdp_do_flush);
>  
> -void bpf_clear_redirect_map(struct bpf_map *map)
> -{
> -	struct bpf_redirect_info *ri;
> -	int cpu;
> -
> -	for_each_possible_cpu(cpu) {
> -		ri = per_cpu_ptr(&bpf_redirect_info, cpu);
> -		/* Avoid polluting remote cacheline due to writes if
> -		 * not needed. Once we pass this test, we need the
> -		 * cmpxchg() to make sure it hasn't been changed in
> -		 * the meantime by remote CPU.
> -		 */
> -		if (unlikely(READ_ONCE(ri->map) == map))
> -			cmpxchg(&ri->map, map, NULL);
> -	}
> -}
> -
>  int xdp_do_redirect(struct net_device *dev, struct xdp_buff *xdp,
>  		    struct bpf_prog *xdp_prog)
>  {
>  	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> -	struct bpf_map *map = READ_ONCE(ri->map);
> -	u32 index = ri->tgt_index;
> +	enum xdp_redirect_type type = ri->tgt_type;
>  	void *fwd = ri->tgt_value;
>  	int err;
>  
> -	ri->tgt_index = 0;
> -	ri->tgt_value = NULL;
> -	WRITE_ONCE(ri->map, NULL);
> +	ri->tgt_type = XDP_REDIR_UNSET;
>  
> -	if (unlikely(!map)) {
> -		fwd = dev_get_by_index_rcu(dev_net(dev), index);
> +	switch (type) {
> +	case XDP_REDIR_DEV_IFINDEX:
> +		fwd = dev_get_by_index_rcu(dev_net(dev), (u32)(long)fwd);
>  		if (unlikely(!fwd)) {
>  			err = -EINVAL;
> -			goto err;
> +			break;
>  		}
> -
>  		err = dev_xdp_enqueue(fwd, xdp, dev);
> -	} else {
> -		err = __bpf_tx_xdp_map(dev, fwd, map, xdp);
> +		break;
> +	case XDP_REDIR_DEV_MAP:
> +		err = dev_map_enqueue(fwd, xdp, dev);
> +		break;
> +	case XDP_REDIR_CPU_MAP:
> +		err = cpu_map_enqueue(fwd, xdp, dev);
> +		break;
> +	case XDP_REDIR_XSK_MAP:
> +		err = __xsk_map_redirect(fwd, xdp);
> +		break;
> +	default:
> +		err = -EBADRQC;
>  	}
>  
>  	if (unlikely(err))
>  		goto err;
>  
> -	_trace_xdp_redirect_map(dev, xdp_prog, fwd, map, index);
> +	_trace_xdp_redirect_map(dev, xdp_prog, fwd, type, ri);
>  	return 0;
>  err:
> -	_trace_xdp_redirect_map_err(dev, xdp_prog, fwd, map, index, err);
> +	_trace_xdp_redirect_map_err(dev, xdp_prog, fwd, type, ri, err);
>  	return err;
>  }
>  EXPORT_SYMBOL_GPL(xdp_do_redirect);
> @@ -4001,41 +3974,40 @@ static int xdp_do_generic_redirect_map(struct net_device *dev,
>  				       struct sk_buff *skb,
>  				       struct xdp_buff *xdp,
>  				       struct bpf_prog *xdp_prog,
> -				       struct bpf_map *map)
> +				       void *fwd,
> +				       enum xdp_redirect_type type)
>  {
>  	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> -	u32 index = ri->tgt_index;
> -	void *fwd = ri->tgt_value;
> -	int err = 0;
> -
> -	ri->tgt_index = 0;
> -	ri->tgt_value = NULL;
> -	WRITE_ONCE(ri->map, NULL);
> +	int err;
>  
> -	if (map->map_type == BPF_MAP_TYPE_DEVMAP ||
> -	    map->map_type == BPF_MAP_TYPE_DEVMAP_HASH) {
> +	switch (type) {
> +	case XDP_REDIR_DEV_MAP: {
>  		struct bpf_dtab_netdev *dst = fwd;

I thought the braces around the case body looked a bit odd. I guess
that's to get a local scope for the dst var (and xs var below), right?
This is basically a cast, though, so I wonder if you couldn't just as
well use the fwd pointer directly (with a cast) in the function call
below? WDYT?

(Strictly speaking I don't think the compiler will even complain if you
omit the cast as well, but having it in there is nice fore readability I
think, and guards against someone forgetting to update the call if the
function prototype changes).

>  		err = dev_map_generic_redirect(dst, skb, xdp_prog);
>  		if (unlikely(err))
>  			goto err;
> -	} else if (map->map_type == BPF_MAP_TYPE_XSKMAP) {
> +		break;
> +	}
> +	case XDP_REDIR_XSK_MAP: {
>  		struct xdp_sock *xs = fwd;
>  
>  		err = xsk_generic_rcv(xs, xdp);
>  		if (err)
>  			goto err;
>  		consume_skb(skb);
> -	} else {
> +		break;
> +	}
> +	default:
>  		/* TODO: Handle BPF_MAP_TYPE_CPUMAP */
>  		err = -EBADRQC;
>  		goto err;
>  	}
>  
> -	_trace_xdp_redirect_map(dev, xdp_prog, fwd, map, index);
> +	_trace_xdp_redirect_map(dev, xdp_prog, fwd, type, ri);
>  	return 0;
>  err:
> -	_trace_xdp_redirect_map_err(dev, xdp_prog, fwd, map, index, err);
> +	_trace_xdp_redirect_map_err(dev, xdp_prog, fwd, type, ri, err);
>  	return err;
>  }
>  
> @@ -4043,29 +4015,31 @@ int xdp_do_generic_redirect(struct net_device *dev, struct sk_buff *skb,
>  			    struct xdp_buff *xdp, struct bpf_prog *xdp_prog)
>  {
>  	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
> -	struct bpf_map *map = READ_ONCE(ri->map);
> -	u32 index = ri->tgt_index;
> -	struct net_device *fwd;
> +	enum xdp_redirect_type type = ri->tgt_type;
> +	void *fwd = ri->tgt_value;
>  	int err = 0;
>  
> -	if (map)
> -		return xdp_do_generic_redirect_map(dev, skb, xdp, xdp_prog,
> -						   map);
> -	ri->tgt_index = 0;
> -	fwd = dev_get_by_index_rcu(dev_net(dev), index);
> -	if (unlikely(!fwd)) {
> -		err = -EINVAL;
> -		goto err;
> -	}
> +	ri->tgt_type = XDP_REDIR_UNSET;
> +	ri->tgt_value = NULL;
>  
> -	err = xdp_ok_fwd_dev(fwd, skb->len);
> -	if (unlikely(err))
> -		goto err;
> +	if (type == XDP_REDIR_DEV_IFINDEX) {
> +		fwd = dev_get_by_index_rcu(dev_net(dev), (u32)(long)fwd);
> +		if (unlikely(!fwd)) {
> +			err = -EINVAL;
> +			goto err;
> +		}
>  
> -	skb->dev = fwd;
> -	_trace_xdp_redirect(dev, xdp_prog, index);
> -	generic_xdp_tx(skb, xdp_prog);
> -	return 0;
> +		err = xdp_ok_fwd_dev(fwd, skb->len);
> +		if (unlikely(err))
> +			goto err;
> +
> +		skb->dev = fwd;
> +		_trace_xdp_redirect(dev, xdp_prog, index);
> +		generic_xdp_tx(skb, xdp_prog);
> +		return 0;
> +	}
> +
> +	return xdp_do_generic_redirect_map(dev, skb, xdp, xdp_prog, fwd, type);
>  err:
>  	_trace_xdp_redirect_err(dev, xdp_prog, index, err);
>  	return err;
> @@ -4078,10 +4052,9 @@ BPF_CALL_2(bpf_xdp_redirect, u32, ifindex, u64, flags)
>  	if (unlikely(flags))
>  		return XDP_ABORTED;
>  
> -	ri->flags = flags;
> -	ri->tgt_index = ifindex;
> -	ri->tgt_value = NULL;
> -	WRITE_ONCE(ri->map, NULL);
> +	ri->tgt_type = XDP_REDIR_DEV_IFINDEX;
> +	ri->tgt_index = 0;
> +	ri->tgt_value = (void *)(long)ifindex;
>  
>  	return XDP_REDIRECT;
>  }
> @@ -4096,7 +4069,8 @@ static const struct bpf_func_proto bpf_xdp_redirect_proto = {
>  
>  static __always_inline s64 __bpf_xdp_redirect_map(struct bpf_map *map, u32 ifindex, u64 flags,
>  						  void *lookup_elem(struct bpf_map *map,
> -								    u32 key))
> +								    u32 key),
> +						  enum xdp_redirect_type type)
>  {
>  	struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>  
> @@ -4105,35 +4079,39 @@ static __always_inline s64 __bpf_xdp_redirect_map(struct bpf_map *map, u32 ifind
>  
>  	ri->tgt_value = lookup_elem(map, ifindex);
>  	if (unlikely(!ri->tgt_value)) {
> -		WRITE_ONCE(ri->map, NULL);
> +		ri->tgt_type = XDP_REDIR_UNSET;
>  		return flags;
>  	}
>  
> -	ri->flags = flags;
>  	ri->tgt_index = ifindex;
> -	WRITE_ONCE(ri->map, map);
> +	ri->tgt_type = type;
> +	ri->map_id = map->id;
>  
>  	return XDP_REDIRECT;
>  }
>  
>  BPF_CALL_3(bpf_xdp_redirect_devmap, struct bpf_map *, map, u32, ifindex, u64, flags)
>  {
> -	return __bpf_xdp_redirect_map(map, ifindex, flags, __dev_map_lookup_elem);
> +	return __bpf_xdp_redirect_map(map, ifindex, flags, __dev_map_lookup_elem,
> +				      XDP_REDIR_DEV_MAP);
>  }
>  
>  BPF_CALL_3(bpf_xdp_redirect_devmap_hash, struct bpf_map *, map, u32, ifindex, u64, flags)
>  {
> -	return __bpf_xdp_redirect_map(map, ifindex, flags, __dev_map_hash_lookup_elem);
> +	return __bpf_xdp_redirect_map(map, ifindex, flags, __dev_map_hash_lookup_elem,
> +				      XDP_REDIR_DEV_MAP);
>  }
>  
>  BPF_CALL_3(bpf_xdp_redirect_cpumap, struct bpf_map *, map, u32, ifindex, u64, flags)
>  {
> -	return __bpf_xdp_redirect_map(map, ifindex, flags, __cpu_map_lookup_elem);
> +	return __bpf_xdp_redirect_map(map, ifindex, flags, __cpu_map_lookup_elem,
> +				      XDP_REDIR_CPU_MAP);
>  }
>  
>  BPF_CALL_3(bpf_xdp_redirect_xskmap, struct bpf_map *, map, u32, ifindex, u64, flags)
>  {
> -	return __bpf_xdp_redirect_map(map, ifindex, flags, __xsk_map_lookup_elem);
> +	return __bpf_xdp_redirect_map(map, ifindex, flags, __xsk_map_lookup_elem,
> +				      XDP_REDIR_XSK_MAP);
>  }
>  
>  bpf_func_proto_func get_xdp_redirect_func(enum bpf_map_type map_type)
> diff --git a/net/xdp/xskmap.c b/net/xdp/xskmap.c
> index 113fd9017203..c285d3dd04ad 100644
> --- a/net/xdp/xskmap.c
> +++ b/net/xdp/xskmap.c
> @@ -87,7 +87,6 @@ static void xsk_map_free(struct bpf_map *map)
>  {
>  	struct xsk_map *m = container_of(map, struct xsk_map, map);
>  
> -	bpf_clear_redirect_map(map);
>  	synchronize_net();
>  	bpf_map_area_free(m);
>  }
> -- 
> 2.27.0