docs/bpf: Update documentation for BTF_KIND_ENUM64 support

Add BTF_KIND_ENUM64 documentation in btf.rst.
Also fixed a typo for section number for BTF_KIND_TYPE_TAG
from 2.2.17 to 2.2.18, and fixed a type size issue for
BTF_KIND_ENUM.

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20220607062724.3728215-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: yonghong-song

Documentation/bpf/btf.rst

@@ -74,7 +74,7 @@ sequentially and type id is assigned to each recognized type starting from id
     #define BTF_KIND_ARRAY          3       /* Array        */
     #define BTF_KIND_STRUCT         4       /* Struct       */
     #define BTF_KIND_UNION          5       /* Union        */
-    #define BTF_KIND_ENUM           6       /* Enumeration  */
+    #define BTF_KIND_ENUM           6       /* Enumeration up to 32-bit values */
     #define BTF_KIND_FWD            7       /* Forward      */
     #define BTF_KIND_TYPEDEF        8       /* Typedef      */
     #define BTF_KIND_VOLATILE       9       /* Volatile     */
@@ -87,6 +87,7 @@ sequentially and type id is assigned to each recognized type starting from id
     #define BTF_KIND_FLOAT          16      /* Floating point       */
     #define BTF_KIND_DECL_TAG       17      /* Decl Tag     */
     #define BTF_KIND_TYPE_TAG       18      /* Type Tag     */
+    #define BTF_KIND_ENUM64         19      /* Enumeration up to 64-bit values */
 
 Note that the type section encodes debug info, not just pure types.
 ``BTF_KIND_FUNC`` is not a type, and it represents a defined subprogram.
@@ -101,10 +102,10 @@ Each type contains the following common data::
          * bits 24-28: kind (e.g. int, ptr, array...etc)
          * bits 29-30: unused
          * bit     31: kind_flag, currently used by
-         *             struct, union and fwd
+         *             struct, union, fwd, enum and enum64.
          */
         __u32 info;
-        /* "size" is used by INT, ENUM, STRUCT and UNION.
+        /* "size" is used by INT, ENUM, STRUCT, UNION and ENUM64.
          * "size" tells the size of the type it is describing.
          *
          * "type" is used by PTR, TYPEDEF, VOLATILE, CONST, RESTRICT,
@@ -281,10 +282,10 @@ modes exist:
 
 ``struct btf_type`` encoding requirement:
   * ``name_off``: 0 or offset to a valid C identifier
-  * ``info.kind_flag``: 0
+  * ``info.kind_flag``: 0 for unsigned, 1 for signed
   * ``info.kind``: BTF_KIND_ENUM
   * ``info.vlen``: number of enum values
-  * ``size``: 4
+  * ``size``: 1/2/4/8
 
 ``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum``.::
 
@@ -297,6 +298,10 @@ The ``btf_enum`` encoding:
   * ``name_off``: offset to a valid C identifier
   * ``val``: any value
 
+If the original enum value is signed and the size is less than 4,
+that value will be sign extended into 4 bytes. If the size is 8,
+the value will be truncated into 4 bytes.
+
 2.2.7 BTF_KIND_FWD
 ~~~~~~~~~~~~~~~~~~
 
@@ -493,7 +498,7 @@ the attribute is applied to a ``struct``/``union`` member or
 a ``func`` argument, and ``btf_decl_tag.component_idx`` should be a
 valid index (starting from 0) pointing to a member or an argument.
 
-2.2.17 BTF_KIND_TYPE_TAG
+2.2.18 BTF_KIND_TYPE_TAG
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
 ``struct btf_type`` encoding requirement:
@@ -516,6 +521,32 @@ type_tag, then zero or more const/volatile/restrict/typedef
 and finally the base type. The base type is one of
 int, ptr, array, struct, union, enum, func_proto and float types.
 
+2.2.19 BTF_KIND_ENUM64
+~~~~~~~~~~~~~~~~~~~~~~
+
+``struct btf_type`` encoding requirement:
+  * ``name_off``: 0 or offset to a valid C identifier
+  * ``info.kind_flag``: 0 for unsigned, 1 for signed
+  * ``info.kind``: BTF_KIND_ENUM64
+  * ``info.vlen``: number of enum values
+  * ``size``: 1/2/4/8
+
+``btf_type`` is followed by ``info.vlen`` number of ``struct btf_enum64``.::
+
+    struct btf_enum64 {
+        __u32   name_off;
+        __u32   val_lo32;
+        __u32   val_hi32;
+    };
+
+The ``btf_enum64`` encoding:
+  * ``name_off``: offset to a valid C identifier
+  * ``val_lo32``: lower 32-bit value for a 64-bit value
+  * ``val_hi32``: high 32-bit value for a 64-bit value
+
+If the original enum value is signed and the size is less than 8,
+that value will be sign extended into 8 bytes.
+
 3. BTF Kernel API
 =================