Add pin support for xsk map (iovisor#4061)

Add pin support for xsk map.

Signed-off-by: Y7n05h <[email protected]>

docs/reference_guide.md

@@ -1145,9 +1145,9 @@ Examples in situ:
 
 ### 13. BPF_XSKMAP
 
-Syntax: ```BPF_XSKMAP(name, size)```
+Syntax: ```BPF_XSKMAP(name, size [, "/sys/fs/bpf/xyz"])```
 
-This creates a xsk map named ```name``` with ```size``` entries. Each entry represents one NIC's queue id. This map is only used in XDP to redirect packet to an AF_XDP socket. If the AF_XDP socket is binded to a queue which is different than the current packet's queue id, the packet will be dropped. For kernel v5.3 and latter, `lookup` method is available and can be used to check whether and AF_XDP socket is available for the current packet's queue id. More details at [AF_XDP](https://www.kernel.org/doc/html/latest/networking/af_xdp.html).
+This creates a xsk map named ```name``` with ```size``` entries and pin it to the bpffs as a FILE. Each entry represents one NIC's queue id. This map is only used in XDP to redirect packet to an AF_XDP socket. If the AF_XDP socket is binded to a queue which is different than the current packet's queue id, the packet will be dropped. For kernel v5.3 and latter, `lookup` method is available and can be used to check whether and AF_XDP socket is available for the current packet's queue id. More details at [AF_XDP](https://www.kernel.org/doc/html/latest/networking/af_xdp.html).
 
 For example:
 ```C

src/cc/export/helpers.h

@@ -390,7 +390,7 @@ struct _name##_table_t _name = { .max_entries = (_max_entries) }
 #define BPF_CPUMAP(_name, _max_entries) \
  BPF_XDP_REDIRECT_MAP("cpumap", u32, _name, _max_entries)
 
-#define BPF_XSKMAP(_name, _max_entries) \
+#define _BPF_XSKMAP(_name, _max_entries, _pinned) \
 struct _name##_table_t { \
  u32 key; \
  int leaf; \
@@ -399,8 +399,12 @@ struct _name##_table_t { \
  u64 (*redirect_map) (int, int); \
  u32 max_entries; \
 }; \
-__attribute__((section("maps/xskmap"))) \
+__attribute__((section("maps/xskmap" _pinned))) \
 struct _name##_table_t _name = { .max_entries = (_max_entries) }
+#define BPF_XSKMAP2(_name, _max_entries) _BPF_XSKMAP(_name, _max_entries, "")
+#define BPF_XSKMAP3(_name, _max_entries, _pinned) _BPF_XSKMAP(_name, _max_entries, ":" _pinned)
+#define BPF_XSKMAPX(_1, _2, _3, NAME, ...) NAME
+#define BPF_XSKMAP(...) BPF_XSKMAPX(__VA_ARGS__, BPF_XSKMAP3, BPF_XSKMAP2)(__VA_ARGS__)
 
 #define BPF_ARRAY_OF_MAPS(_name, _inner_map_name, _max_entries) \
  BPF_TABLE("array_of_maps$" _inner_map_name, int, int, _name, _max_entries)