bpf, docs: Add extended call instructions

Add extended call instructions.  Uses the term "program-local" for
call by offset.  And there are instructions for calling helper functions
by "address" (the old way of using integer values), and for calling
helper functions by BTF ID (for kfuncs).

V1 -> V2: addressed comments from David Vernet

V2 -> V3: make descriptions in table consistent with updated names

V3 -> V4: addressed comments from Alexei

V4 -> V5: fixed alignment
Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230326033117.1075-1-dthaler1968@googlemail.comSigned-off-by: Alexei Starovoitov <ast@kernel.org>

Documentation/bpf/instruction-set.rst

@@ -243,27 +243,29 @@ Jump instructions
 otherwise identical operations.
 The 'code' field encodes the operation as below:
 
-========  =====  =========================  ============
-code      value  description                notes
-========  =====  =========================  ============
-BPF_JA    0x00   PC += off                  BPF_JMP only
-BPF_JEQ   0x10   PC += off if dst == src
-BPF_JGT   0x20   PC += off if dst > src     unsigned
-BPF_JGE   0x30   PC += off if dst >= src    unsigned
-BPF_JSET  0x40   PC += off if dst & src
-BPF_JNE   0x50   PC += off if dst != src
-BPF_JSGT  0x60   PC += off if dst > src     signed
-BPF_JSGE  0x70   PC += off if dst >= src    signed
-BPF_CALL  0x80   function call              see `Helper functions`_
-BPF_EXIT  0x90   function / program return  BPF_JMP only
-BPF_JLT   0xa0   PC += off if dst < src     unsigned
-BPF_JLE   0xb0   PC += off if dst <= src    unsigned
-BPF_JSLT  0xc0   PC += off if dst < src     signed
-BPF_JSLE  0xd0   PC += off if dst <= src    signed
-========  =====  =========================  ============
+========  =====  ===  ===========================================  =========================================
+code      value  src  description                                  notes
+========  =====  ===  ===========================================  =========================================
+BPF_JA    0x0    0x0  PC += offset                                 BPF_JMP only
+BPF_JEQ   0x1    any  PC += offset if dst == src
+BPF_JGT   0x2    any  PC += offset if dst > src                    unsigned
+BPF_JGE   0x3    any  PC += offset if dst >= src                   unsigned
+BPF_JSET  0x4    any  PC += offset if dst & src
+BPF_JNE   0x5    any  PC += offset if dst != src
+BPF_JSGT  0x6    any  PC += offset if dst > src                    signed
+BPF_JSGE  0x7    any  PC += offset if dst >= src                   signed
+BPF_CALL  0x8    0x0  call helper function by address              see `Helper functions`_
+BPF_CALL  0x8    0x1  call PC += offset                            see `Program-local functions`_
+BPF_CALL  0x8    0x2  call helper function by BTF ID               see `Helper functions`_
+BPF_EXIT  0x9    0x0  return                                       BPF_JMP only
+BPF_JLT   0xa    any  PC += offset if dst < src                    unsigned
+BPF_JLE   0xb    any  PC += offset if dst <= src                   unsigned
+BPF_JSLT  0xc    any  PC += offset if dst < src                    signed
+BPF_JSLE  0xd    any  PC += offset if dst <= src                   signed
+========  =====  ===  ===========================================  =========================================
 
 The eBPF program needs to store the return value into register R0 before doing a
-BPF_EXIT.
+``BPF_EXIT``.
 
 Example:
 
@@ -277,9 +279,22 @@ Helper functions
 ~~~~~~~~~~~~~~~~
 
 Helper functions are a concept whereby BPF programs can call into a
-set of function calls exposed by the runtime.  Each helper
-function is identified by an integer used in a ``BPF_CALL`` instruction.
-The available helper functions may differ for each program type.
+set of function calls exposed by the underlying platform.
+
+Historically, each helper function was identified by an address
+encoded in the imm field.  The available helper functions may differ
+for each program type, but address values are unique across all program types.
+
+Platforms that support the BPF Type Format (BTF) support identifying
+a helper function by a BTF ID encoded in the imm field, where the BTF ID
+identifies the helper name and type.
+
+Program-local functions
+~~~~~~~~~~~~~~~~~~~~~~~
+Program-local functions are functions exposed by the same BPF program as the
+caller, and are referenced by offset from the call instruction, similar to
+``BPF_JA``.  A ``BPF_EXIT`` within the program-local function will return to
+the caller.
 
 Load and store instructions
 ===========================