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docs/bpf: Add documentation for new instructions 

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.

Cc: bpf@ietf.org
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230728011342.3724411-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Yonghong Song 2023-07-27 18:13:42 -07:00 committed by Alexei Starovoitov
parent 0c606571ae
commit 245d4c40c0
2 changed files with 79 additions and 41 deletions
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5
Documentation/bpf/bpf_design_QA.rst
View File

@ -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

115
Documentation/bpf/standardization/instruction-set.rst
View File

@ -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 = -dst
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 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 = -dst
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,s16,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 +201,20 @@ 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 ``BPF_ALU``, 'imm' is
interpreted as a signed value. For ``BPF_ALU64``, the 'imm' is sign extended
from 32 to 64 and interpreted as a signed 64-bit value.
Instruction BPF_MOVSX does move operation with sign extension.
``BPF_ALU | MOVSX`` sign extendes 8-bit and 16-bit into 32-bit and upper 32-bit are zeroed.
``BPF_ALU64 | MOVSX`` sign extends 8-bit, 16-bit and 32-bit into 64-bit.
Shift operations use a mask of 0x3F (63) for 64-bit operations and 0x1F (31)
for 32-bit operations.
@ -210,21 +222,23 @@ 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 and must be 0.
========= ===== =================================================
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 +253,12 @@ Examples:
dst = htobe64(dst)
``BPF_ALU64 | BPF_TO_LE | BPF_END`` with imm = 16/32/64 means::
dst = bswap16 dst
dst = bswap32 dst
dst = bswap64 dst
Jump instructions
-----------------
@ -249,7 +269,8 @@ 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 +299,16 @@ Example:
where 's>=' indicates a signed '>=' comparison.
``BPF_JA | BPF_K | BPF_JMP32`` (0x06) means::
gotol +imm
where 'imm' means the branch offset comes from insn 'imm' field.
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 +351,7 @@ 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`_
============= ===== ==================================== =============
@ -350,9 +382,20 @@ instructions that transfer data between a register and memory.
``BPF_MEM | <size> | BPF_LDX`` means::
dst = *(size *) (src + offset)
dst = *(unsigned size *) (src + offset)
Where size is one of: ``BPF_B``, ``BPF_H``, ``BPF_W``, or ``BPF_DW``.
Where size is one of: ``BPF_B``, ``BPF_H``, ``BPF_W``, or ``BPF_DW`` and
'unsigned size' is one of u8, u16, u32 and u64.
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 = *(signed size *) (src + offset)
Where size is one of: ``BPF_B``, ``BPF_H`` or ``BPF_W``, and
'signed size' is one of s8, s16 and s32.
Atomic operations
-----------------