From a92adde8d3d466e4d945bc4b684795543d0b6594 Mon Sep 17 00:00:00 2001 From: Dave Thaler Date: Fri, 27 Jan 2023 22:45:55 +0000 Subject: [PATCH] bpf, docs: Use consistent names for the same field Use consistent names for the same field, e.g., 'dst' vs 'dst_reg'. Previously a mix of terms were used for the same thing in various cases. Signed-off-by: Dave Thaler Acked-by: David Vernet Link: https://lore.kernel.org/r/20230127224555.916-1-dthaler1968@googlemail.com Signed-off-by: Alexei Starovoitov --- Documentation/bpf/instruction-set.rst | 113 ++++++++++++++++++-------- 1 file changed, 77 insertions(+), 36 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index 2d3fe59bd260..c1c17ee60ecb 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -30,20 +30,56 @@ Instruction encoding eBPF has two instruction encodings: * the basic instruction encoding, which uses 64 bits to encode an instruction -* the wide instruction encoding, which appends a second 64-bit immediate value - (imm64) after the basic instruction for a total of 128 bits. +* the wide instruction encoding, which appends a second 64-bit immediate (i.e., + constant) value after the basic instruction for a total of 128 bits. -The basic instruction encoding looks as follows: +The basic instruction encoding is as follows, where MSB and LSB mean the most significant +bits and least significant bits, respectively: -============= ======= =============== ==================== ============ -32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB) -============= ======= =============== ==================== ============ -immediate offset source register destination register opcode -============= ======= =============== ==================== ============ +============= ======= ======= ======= ============ +32 bits (MSB) 16 bits 4 bits 4 bits 8 bits (LSB) +============= ======= ======= ======= ============ +imm offset src_reg dst_reg opcode +============= ======= ======= ======= ============ + +**imm** + signed integer immediate value + +**offset** + signed integer offset used with pointer arithmetic + +**src_reg** + the source register number (0-10), except where otherwise specified + (`64-bit immediate instructions`_ reuse this field for other purposes) + +**dst_reg** + destination register number (0-10) + +**opcode** + operation to perform Note that most instructions do not use all of the fields. Unused fields shall be cleared to zero. +As discussed below in `64-bit immediate instructions`_, a 64-bit immediate +instruction uses a 64-bit immediate value that is constructed as follows. +The 64 bits following the basic instruction contain a pseudo instruction +using the same format but with opcode, dst_reg, src_reg, and offset all set to zero, +and imm containing the high 32 bits of the immediate value. + +================= ================== +64 bits (MSB) 64 bits (LSB) +================= ================== +basic instruction pseudo instruction +================= ================== + +Thus the 64-bit immediate value is constructed as follows: + + imm64 = (next_imm << 32) | imm + +where 'next_imm' refers to the imm value of the pseudo instruction +following the basic instruction. + Instruction classes ------------------- @@ -71,27 +107,32 @@ For arithmetic and jump instructions (``BPF_ALU``, ``BPF_ALU64``, ``BPF_JMP`` an ============== ====== ================= 4 bits (MSB) 1 bit 3 bits (LSB) ============== ====== ================= -operation code source instruction class +code source instruction class ============== ====== ================= -The 4th bit encodes the source operand: +**code** + the operation code, whose meaning varies by instruction class - ====== ===== ======================================== +**source** + the source operand location, which unless otherwise specified is one of: + + ====== ===== ============================================== source value description - ====== ===== ======================================== - BPF_K 0x00 use 32-bit immediate as source operand - BPF_X 0x08 use 'src_reg' register as source operand - ====== ===== ======================================== - -The four MSB bits store the operation code. + ====== ===== ============================================== + BPF_K 0x00 use 32-bit 'imm' value as source operand + BPF_X 0x08 use 'src_reg' register value as source operand + ====== ===== ============================================== +**instruction class** + the instruction class (see `Instruction classes`_) Arithmetic instructions ----------------------- ``BPF_ALU`` uses 32-bit wide operands while ``BPF_ALU64`` uses 64-bit wide operands for otherwise identical operations. -The 'code' field encodes the operation as below: +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 @@ -121,19 +162,19 @@ the destination register is unchanged whereas for ``BPF_ALU`` the upper ``BPF_ADD | BPF_X | BPF_ALU`` means:: - dst_reg = (u32) dst_reg + (u32) src_reg; + dst = (u32) ((u32) dst + (u32) src) ``BPF_ADD | BPF_X | BPF_ALU64`` means:: - dst_reg = dst_reg + src_reg + dst = dst + src ``BPF_XOR | BPF_K | BPF_ALU`` means:: - dst_reg = (u32) dst_reg ^ (u32) imm32 + dst = (u32) dst ^ (u32) imm32 ``BPF_XOR | BPF_K | BPF_ALU64`` means:: - dst_reg = dst_reg ^ imm32 + 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 @@ -167,11 +208,11 @@ Examples: ``BPF_ALU | BPF_TO_LE | BPF_END`` with imm = 16 means:: - dst_reg = htole16(dst_reg) + dst = htole16(dst) ``BPF_ALU | BPF_TO_BE | BPF_END`` with imm = 64 means:: - dst_reg = htobe64(dst_reg) + dst = htobe64(dst) Jump instructions ----------------- @@ -246,15 +287,15 @@ instructions that transfer data between a register and memory. ``BPF_MEM | | BPF_STX`` means:: - *(size *) (dst_reg + off) = src_reg + *(size *) (dst + offset) = src ``BPF_MEM | | BPF_ST`` means:: - *(size *) (dst_reg + off) = imm32 + *(size *) (dst + offset) = imm32 ``BPF_MEM | | BPF_LDX`` means:: - dst_reg = *(size *) (src_reg + off) + dst = *(size *) (src + offset) Where size is one of: ``BPF_B``, ``BPF_H``, ``BPF_W``, or ``BPF_DW``. @@ -288,11 +329,11 @@ BPF_XOR 0xa0 atomic xor ``BPF_ATOMIC | BPF_W | BPF_STX`` with 'imm' = BPF_ADD means:: - *(u32 *)(dst_reg + off16) += src_reg + *(u32 *)(dst + offset) += src ``BPF_ATOMIC | BPF_DW | BPF_STX`` with 'imm' = BPF ADD means:: - *(u64 *)(dst_reg + off16) += src_reg + *(u64 *)(dst + offset) += src In addition to the simple atomic operations, there also is a modifier and two complex atomic operations: @@ -307,16 +348,16 @@ BPF_CMPXCHG 0xf0 | BPF_FETCH atomic compare and exchange The ``BPF_FETCH`` modifier is optional for simple atomic operations, and always set for the complex atomic operations. If the ``BPF_FETCH`` flag -is set, then the operation also overwrites ``src_reg`` with the value that +is set, then the operation also overwrites ``src`` with the value that was in memory before it was modified. -The ``BPF_XCHG`` operation atomically exchanges ``src_reg`` with the value -addressed by ``dst_reg + off``. +The ``BPF_XCHG`` operation atomically exchanges ``src`` with the value +addressed by ``dst + offset``. The ``BPF_CMPXCHG`` operation atomically compares the value addressed by -``dst_reg + off`` with ``R0``. If they match, the value addressed by -``dst_reg + off`` is replaced with ``src_reg``. In either case, the -value that was at ``dst_reg + off`` before the operation is zero-extended +``dst + offset`` with ``R0``. If they match, the value addressed by +``dst + offset`` is replaced with ``src``. In either case, the +value that was at ``dst + offset`` before the operation is zero-extended and loaded back to ``R0``. 64-bit immediate instructions @@ -329,7 +370,7 @@ There is currently only one such instruction. ``BPF_LD | BPF_DW | BPF_IMM`` means:: - dst_reg = imm64 + dst = imm64 Legacy BPF Packet access instructions