Use consistent terminology and structured RST elements to better document these two oddball instructions. Signed-off-by: Christoph Hellwig <hch@xxxxxx> --- Documentation/bpf/instruction-set.rst | 54 ++++++++++++++++----------- 1 file changed, 32 insertions(+), 22 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index 03da885301722..b3c2621216c97 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -213,8 +213,8 @@ The mode modifier is one of: mode modifier value description ============= ===== ==================================== BPF_IMM 0x00 used for 64-bit mov - BPF_ABS 0x20 legacy BPF packet access - BPF_IND 0x40 legacy BPF packet access + BPF_ABS 0x20 legacy BPF packet access (absolute) + BPF_IND 0x40 legacy BPF packet access (indirect) BPF_MEM 0x60 regular load and store operations BPF_ATOMIC 0xc0 atomic operations ============= ===== ==================================== @@ -294,29 +294,39 @@ eBPF has one 16-byte instruction: ``BPF_LD | BPF_DW | BPF_IMM`` which consists of two consecutive ``struct bpf_insn`` 8-byte blocks and interpreted as single instruction that loads 64-bit immediate value into a dst_reg. -Packet access instructions --------------------------- +Legacy BPF Packet access instructions +------------------------------------- -eBPF has two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and -(BPF_IND | <size> | BPF_LD) which are used to access packet data. +eBPF has special instructions for access to packet data that have been +carried over from classic BPF to retain the performance of legacy socket +filters running in the eBPF interpreter. -They had to be carried over from classic BPF to have strong performance of -socket filters running in eBPF interpreter. These instructions can only -be used when interpreter context is a pointer to ``struct sk_buff`` and -have seven implicit operands. Register R6 is an implicit input that must -contain pointer to sk_buff. Register R0 is an implicit output which contains -the data fetched from the packet. Registers R1-R5 are scratch registers -and must not be used to store the data across BPF_ABS | BPF_LD or -BPF_IND | BPF_LD instructions. +The instructions come in two forms: ``BPF_ABS | <size> | BPF_LD`` and +``BPF_IND | <size> | BPF_LD``. -These instructions have implicit program exit condition as well. When -eBPF program is trying to access the data beyond the packet boundary, -the interpreter will abort the execution of the program. JIT compilers -therefore must preserve this property. src_reg and imm32 fields are -explicit inputs to these instructions. +These instructions are used to access packet data and can only be used when +the interpreter context is a pointer to networking packet. ``BPF_ABS`` +accesses packet data at an absolute offset specified by the immediate data +and ``BPF_IND`` access packet data at an offset that includes the value of +a register in addition to the immediate data. -For example, BPF_IND | BPF_W | BPF_LD means:: +These instructions have seven implicit operands: - R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32)) + * Register R6 is an implicit input that must contain pointer to a + struct sk_buff. + * Register R0 is an implicit output which contains the data fetched from + the packet. + * Registers R1-R5 are scratch registers that are clobbered after a call to + ``BPF_ABS | BPF_LD`` or ``BPF_IND`` | BPF_LD instructions. + +These instructions have an implicit program exit condition as well. When an +eBPF program is trying to access the data beyond the packet boundary, the +interpreter will abort the execution of the program. + +``BPF_ABS | BPF_W | BPF_LD`` means:: -and R1 - R5 are clobbered. + R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + imm32)) + +``BPF_IND | BPF_W | BPF_LD`` means:: + + R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32)) -- 2.30.2
