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bpf, docs: Move the packet access instructions last in instruction-set.rst

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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@lst.de>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211223101906.977624-5-hch@lst.de
This commit is contained in:
Christoph Hellwig 2021-12-23 11:19:06 +01:00 committed by Alexei Starovoitov
parent 5e4dd19f00
commit 63d000c3dc
1 changed files with 27 additions and 28 deletions
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55
Documentation/bpf/instruction-set.rst
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@ -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.