[PATCH bpf-next v2 15/15] docs/bpf: Add documentation for new instructions

[Yonghong Song <yhs@fb.com>]

Add documentation in instruction-set.rst for new instruction encoding
and their corresponding operations. Also removed the question
related to 'no BPF_SDIV' in bpf_design_QA.rst since we have
BPF_SDIV insn now.

Signed-off-by: Yonghong Song <yhs@fb.com>
---
 Documentation/bpf/bpf_design_QA.rst           |   5 -
 .../bpf/standardization/instruction-set.rst   | 100 ++++++++++++------
 2 files changed, 66 insertions(+), 39 deletions(-)

diff --git a/Documentation/bpf/bpf_design_QA.rst b/Documentation/bpf/bpf_design_QA.rst
index 38372a956d65..eb19c945f4d5 100644
--- a/Documentation/bpf/bpf_design_QA.rst
+++ b/Documentation/bpf/bpf_design_QA.rst
@@ -140,11 +140,6 @@ A: Because if we picked one-to-one relationship to x64 it would have made
 it more complicated to support on arm64 and other archs. Also it
 needs div-by-zero runtime check.
 
-Q: Why there is no BPF_SDIV for signed divide operation?
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-A: Because it would be rarely used. llvm errors in such case and
-prints a suggestion to use unsigned divide instead.
-
 Q: Why BPF has implicit prologue and epilogue?
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 A: Because architectures like sparc have register windows and in general
diff --git a/Documentation/bpf/standardization/instruction-set.rst b/Documentation/bpf/standardization/instruction-set.rst
index 751e657973f0..367f426d09a1 100644
--- a/Documentation/bpf/standardization/instruction-set.rst
+++ b/Documentation/bpf/standardization/instruction-set.rst
@@ -154,24 +154,27 @@ otherwise identical operations.
 The 'code' field encodes the operation as below, where 'src' and 'dst' refer
 to the values of the source and destination registers, respectively.
 
-========  =====  ==========================================================
-code      value  description
-========  =====  ==========================================================
-BPF_ADD   0x00   dst += src
-BPF_SUB   0x10   dst -= src
-BPF_MUL   0x20   dst \*= src
-BPF_DIV   0x30   dst = (src != 0) ? (dst / src) : 0
-BPF_OR    0x40   dst \|= src
-BPF_AND   0x50   dst &= src
-BPF_LSH   0x60   dst <<= (src & mask)
-BPF_RSH   0x70   dst >>= (src & mask)
-BPF_NEG   0x80   dst = -src
-BPF_MOD   0x90   dst = (src != 0) ? (dst % src) : dst
-BPF_XOR   0xa0   dst ^= src
-BPF_MOV   0xb0   dst = src
-BPF_ARSH  0xc0   sign extending dst >>= (src & mask)
-BPF_END   0xd0   byte swap operations (see `Byte swap instructions`_ below)
-========  =====  ==========================================================
+========  =====  ============  ==========================================================
+code      value  offset value  description
+========  =====  ============  ==========================================================
+BPF_ADD   0x00   0             dst += src
+BPF_SUB   0x10   0             dst -= src
+BPF_MUL   0x20   0             dst \*= src
+BPF_DIV   0x30   0             dst = (src != 0) ? (dst / src) : 0
+BPF_SDIV  0x30   1             dst = (src != 0) ? (dst s/ src) : 0
+BPF_OR    0x40   0             dst \|= src
+BPF_AND   0x50   0             dst &= src
+BPF_LSH   0x60   0             dst <<= (src & mask)
+BPF_RSH   0x70   0             dst >>= (src & mask)
+BPF_NEG   0x80   0             dst = -src
+BPF_MOD   0x90   0             dst = (src != 0) ? (dst % src) : dst
+BPF_SMOD  0x90   1             dst = (src != 0) ? (dst s% src) : dst
+BPF_XOR   0xa0   0             dst ^= src
+BPF_MOV   0xb0   0             dst = src
+BPF_MOVSX 0xb0   8/16/32       dst = (s8,16,s32)src
+BPF_ARSH  0xc0   0             sign extending dst >>= (src & mask)
+BPF_END   0xd0   0             byte swap operations (see `Byte swap instructions`_ below)
+========  =====  ============  ==========================================================
 
 Underflow and overflow are allowed during arithmetic operations, meaning
 the 64-bit or 32-bit value will wrap. If eBPF program execution would
@@ -198,11 +201,19 @@ where '(u32)' indicates that the upper 32 bits are zeroed.
 
   dst = dst ^ imm32
 
-Also note that the division and modulo operations are unsigned. Thus, for
-``BPF_ALU``, 'imm' is first interpreted as an unsigned 32-bit value, whereas
-for ``BPF_ALU64``, 'imm' is first sign extended to 64 bits and the result
-interpreted as an unsigned 64-bit value. There are no instructions for
-signed division or modulo.
+Note that most instructions have instruction offset of 0. But three instructions
+(BPF_SDIV, BPF_SMOD, BPF_MOVSX) have non-zero offset.
+
+The devision and modulo operations support both unsigned and signed flavors.
+For unsigned operation (BPF_DIV and BPF_MOD), for ``BPF_ALU``, 'imm' is first
+interpreted as an unsigned 32-bit value, whereas for ``BPF_ALU64``, 'imm' is
+first sign extended to 64 bits and the result interpreted as an unsigned 64-bit
+value.  For signed operation (BPF_SDIV and BPF_SMOD), for both ``BPF_ALU`` and
+``BPF_ALU64``, 'imm' is interpreted as a signed value.
+
+Instruction BPF_MOVSX does move operation with sign extension. For ``BPF_ALU``
+mode, 8-bit and 16-bit sign extensions to 32-bit are supported. For ``BPF_ALU64``,
+8-bit, 16-bit and 32-bit sign extenstions to 64-bit are supported.
 
 Shift operations use a mask of 0x3F (63) for 64-bit operations and 0x1F (31)
 for 32-bit operations.
@@ -210,21 +221,23 @@ for 32-bit operations.
 Byte swap instructions
 ~~~~~~~~~~~~~~~~~~~~~~
 
-The byte swap instructions use an instruction class of ``BPF_ALU`` and a 4-bit
-'code' field of ``BPF_END``.
+The byte swap instructions use instruction classes of ``BPF_ALU`` and ``BPF_ALU64``
+and a 4-bit 'code' field of ``BPF_END``.
 
 The byte swap instructions operate on the destination register
 only and do not use a separate source register or immediate value.
 
-The 1-bit source operand field in the opcode is used to select what byte
-order the operation convert from or to:
+For ``BPF_ALU``, the 1-bit source operand field in the opcode is used to select what byte
+order the operation convert from or to. For ``BPF_ALU64``, the 1-bit source operand
+field in the opcode is not used.
 
-=========  =====  =================================================
-source     value  description
-=========  =====  =================================================
-BPF_TO_LE  0x00   convert between host byte order and little endian
-BPF_TO_BE  0x08   convert between host byte order and big endian
-=========  =====  =================================================
+=========  =========  =====  =================================================
+class      source     value  description
+=========  =========  =====  =================================================
+BPF_ALU    BPF_TO_LE  0x00   convert between host byte order and little endian
+BPF_ALU    BPF_TO_BE  0x08   convert between host byte order and big endian
+BPF_ALU64  BPF_TO_LE  0x00   do byte swap unconditionally
+=========  =========  =====  =================================================
 
 The 'imm' field encodes the width of the swap operations.  The following widths
 are supported: 16, 32 and 64.
@@ -239,6 +252,10 @@ Examples:
 
   dst = htobe64(dst)
 
+``BPF_ALU64 | BPF_TO_LE | BPF_END`` with imm = 16 means::
+
+  dst = bswap16(dst)
+
 Jump instructions
 -----------------
 
@@ -249,7 +266,8 @@ The 'code' field encodes the operation as below:
 ========  =====  ===  ===========================================  =========================================
 code      value  src  description                                  notes
 ========  =====  ===  ===========================================  =========================================
-BPF_JA    0x0    0x0  PC += offset                                 BPF_JMP only
+BPF_JA    0x0    0x0  PC += offset                                 BPF_JMP class
+BPF_JA    0x0    0x0  PC += imm                                    BPF_JMP32 class
 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
@@ -278,6 +296,10 @@ Example:
 
 where 's>=' indicates a signed '>=' comparison.
 
+Note there are two flavors of BPF_JA instrions. BPF_JMP class permits 16-bit jump offset while
+BPF_JMP32 permits 32-bit jump offset. A >16bit conditional jmp can be converted to a <16bit
+conditional jmp plus a 32-bit unconditional jump.
+
 Helper functions
 ~~~~~~~~~~~~~~~~
 
@@ -320,6 +342,7 @@ The mode modifier is one of:
   BPF_ABS        0x20   legacy BPF packet access (absolute)   `Legacy BPF Packet access instructions`_
   BPF_IND        0x40   legacy BPF packet access (indirect)   `Legacy BPF Packet access instructions`_
   BPF_MEM        0x60   regular load and store operations     `Regular load and store operations`_
+  BPF_MEMSX      0x80   sign-extension load operations        `Sign-extension load operations`_
   BPF_ATOMIC     0xc0   atomic operations                     `Atomic operations`_
   =============  =====  ====================================  =============
 
@@ -354,6 +377,15 @@ instructions that transfer data between a register and memory.
 
 Where size is one of: ``BPF_B``, ``BPF_H``, ``BPF_W``, or ``BPF_DW``.
 
+The ``BPF_MEMSX`` mode modifier is used to encode sign-extension load
+instructions that transfer data between a register and memory.
+
+``BPF_MEMSX | <size> | BPF_LDX`` means::
+
+  dst = *(sign-extension size *) (src + offset)
+
+Where size is one of: ``BPF_B``, ``BPF_H`` or ``BPF_W``.
+
 Atomic operations
 -----------------
 
-- 
2.34.1