The packet access instructions are a convoluted leftover from classic BPF. Move them last past the much more important atomic operations, and improve the rendering of the code example. Signed-off-by: Christoph Hellwig <hch@xxxxxx> --- Documentation/bpf/instruction-set.rst | 55 +++++++++++++-------------- 1 file changed, 27 insertions(+), 28 deletions(-) diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst index 4e3041cf04325..922635f0c18b7 100644 --- a/Documentation/bpf/instruction-set.rst +++ b/Documentation/bpf/instruction-set.rst @@ -171,34 +171,6 @@ BPF_MEM | <size> | BPF_LDX means:: Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW. -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. - -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. - -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. - -For example:: - - BPF_IND | BPF_W | BPF_LD means: - - R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32)) - and R1 - R5 were scratched. - Atomic operations ----------------- @@ -252,3 +224,30 @@ zero. 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 +-------------------------- + +eBPF has two non-generic instructions: (BPF_ABS | <size> | BPF_LD) and +(BPF_IND | <size> | BPF_LD) which are used to access packet data. + +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. + +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. + +For example, BPF_IND | BPF_W | BPF_LD means:: + + R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32)) + +and R1 - R5 are clobbered. -- 2.30.2
