improve the eBPF documentation v2

improve the eBPF documentation v2

[Christoph Hellwig]

Hi all,

this series first splits the eBPF introductions from the
instruction set document to then drop the classic BPF
documentation from this file to focus on eBPF and to
cleanup the structure and rendering of this file a bit.

Changes since v1:
 - rename intro.rst to classic_vs_extended.rst
 - keep the class vs eBPF tabls in classic_vs_extended.rst
 - a few more editorial tidyups
 - move the packet access instructions last in instruction-set.rst

Diffstat:
 classic_vs_extended.rst |  376 +++++++++++++++++++++++++++++++++++
 index.rst               |    1 
 instruction-set.rst     |  514 ++++++++++++++----------------------------------
 3 files changed, 527 insertions(+), 364 deletions(-)

[PATCH 1/4] bpf, docs: Fix verifier references

[Christoph Hellwig]

Use normal RST file reference instead of linkage copied from the old filter.rst
document that does not actually work when using HTML output.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 Documentation/bpf/instruction-set.rst | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index fa7cba59031e5..fa469078301be 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -203,7 +203,7 @@ Some core changes of the eBPF format from classic BPF:
     bpf_exit
 
   After the call the registers R1-R5 contain junk values and cannot be read.
-  An in-kernel `eBPF verifier`_ is used to validate eBPF programs.
+  An in-kernel verifier.rst is used to validate eBPF programs.
 
 Also in the new design, eBPF is limited to 4096 insns, which means that any
 program will terminate quickly and will only call a fixed number of kernel
@@ -234,7 +234,7 @@ optimizations, socket filters and seccomp are using it as assembler. Tracing
 filters may use it as assembler to generate code from kernel. In kernel usage
 may not be bounded by security considerations, since generated eBPF code
 may be optimizing internal code path and not being exposed to the user space.
-Safety of eBPF can come from the `eBPF verifier`_. In such use cases as
+Safety of eBPF can come from the verifier.rst. In such use cases as
 described, it may be used as safe instruction set.
 
 Just like the original BPF, eBPF runs within a controlled environment,
@@ -462,6 +462,3 @@ of two consecutive ``struct bpf_insn`` 8-byte blocks and interpreted as single
 instruction that loads 64-bit immediate value into a dst_reg.
 Classic BPF has similar instruction: ``BPF_LD | BPF_W | BPF_IMM`` which loads
 32-bit immediate value into a register.
-
-.. Links:
-.. _eBPF verifier: verifiers.rst
-- 
2.30.2

[PATCH 2/4] bpf, docs: Split the comparism to classic BPF from instruction-set.rst

[Christoph Hellwig]

Split the introductory that explain eBPF vs classic BPF and how it maps
to hardware from the instruction set specification into a standalone
document.  This duplicates a little bit of information but gives us a
useful reference for the eBPF instrution set that is not encumbered by
classic BPF.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 Documentation/bpf/classic_vs_extended.rst | 376 +++++++++++++++++++++
 Documentation/bpf/index.rst               |   1 +
 Documentation/bpf/instruction-set.rst     | 380 ++++------------------
 3 files changed, 446 insertions(+), 311 deletions(-)
 create mode 100644 Documentation/bpf/classic_vs_extended.rst

diff --git a/Documentation/bpf/classic_vs_extended.rst b/Documentation/bpf/classic_vs_extended.rst
new file mode 100644
index 0000000000000..2f81a81f5267f
--- /dev/null
+++ b/Documentation/bpf/classic_vs_extended.rst
@@ -0,0 +1,376 @@
+
+===================
+Classic BPF vs eBPF
+===================
+
+eBPF is designed to be JITed with one to one mapping, which can also open up
+the possibility for GCC/LLVM compilers to generate optimized eBPF code through
+an eBPF backend that performs almost as fast as natively compiled code.
+
+Some core changes of the eBPF format from classic BPF:
+
+- Number of registers increase from 2 to 10:
+
+  The old format had two registers A and X, and a hidden frame pointer. The
+  new layout extends this to be 10 internal registers and a read-only frame
+  pointer. Since 64-bit CPUs are passing arguments to functions via registers
+  the number of args from eBPF program to in-kernel function is restricted
+  to 5 and one register is used to accept return value from an in-kernel
+  function. Natively, x86_64 passes first 6 arguments in registers, aarch64/
+  sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved
+  registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers.
+
+  Thus, all eBPF registers map one to one to HW registers on x86_64, aarch64,
+  etc, and eBPF calling convention maps directly to ABIs used by the kernel on
+  64-bit architectures.
+
+  On 32-bit architectures JIT may map programs that use only 32-bit arithmetic
+  and may let more complex programs to be interpreted.
+
+  R0 - R5 are scratch registers and eBPF program needs spill/fill them if
+  necessary across calls. Note that there is only one eBPF program (== one
+  eBPF main routine) and it cannot call other eBPF functions, it can only
+  call predefined in-kernel functions, though.
+
+- Register width increases from 32-bit to 64-bit:
+
+  Still, the semantics of the original 32-bit ALU operations are preserved
+  via 32-bit subregisters. All eBPF registers are 64-bit with 32-bit lower
+  subregisters that zero-extend into 64-bit if they are being written to.
+  That behavior maps directly to x86_64 and arm64 subregister definition, but
+  makes other JITs more difficult.
+
+  32-bit architectures run 64-bit eBPF programs via interpreter.
+  Their JITs may convert BPF programs that only use 32-bit subregisters into
+  native instruction set and let the rest being interpreted.
+
+  Operation is 64-bit, because on 64-bit architectures, pointers are also
+  64-bit wide, and we want to pass 64-bit values in/out of kernel functions,
+  so 32-bit eBPF registers would otherwise require to define register-pair
+  ABI, thus, there won't be able to use a direct eBPF register to HW register
+  mapping and JIT would need to do combine/split/move operations for every
+  register in and out of the function, which is complex, bug prone and slow.
+  Another reason is the use of atomic 64-bit counters.
+
+- Conditional jt/jf targets replaced with jt/fall-through:
+
+  While the original design has constructs such as ``if (cond) jump_true;
+  else jump_false;``, they are being replaced into alternative constructs like
+  ``if (cond) jump_true; /* else fall-through */``.
+
+- Introduces bpf_call insn and register passing convention for zero overhead
+  calls from/to other kernel functions:
+
+  Before an in-kernel function call, the eBPF program needs to
+  place function arguments into R1 to R5 registers to satisfy calling
+  convention, then the interpreter will take them from registers and pass
+  to in-kernel function. If R1 - R5 registers are mapped to CPU registers
+  that are used for argument passing on given architecture, the JIT compiler
+  doesn't need to emit extra moves. Function arguments will be in the correct
+  registers and BPF_CALL instruction will be JITed as single 'call' HW
+  instruction. This calling convention was picked to cover common call
+  situations without performance penalty.
+
+  After an in-kernel function call, R1 - R5 are reset to unreadable and R0 has
+  a return value of the function. Since R6 - R9 are callee saved, their state
+  is preserved across the call.
+
+  For example, consider three C functions::
+
+    u64 f1() { return (*_f2)(1); }
+    u64 f2(u64 a) { return f3(a + 1, a); }
+    u64 f3(u64 a, u64 b) { return a - b; }
+
+  GCC can compile f1, f3 into x86_64::
+
+    f1:
+	movl $1, %edi
+	movq _f2(%rip), %rax
+	jmp  *%rax
+    f3:
+	movq %rdi, %rax
+	subq %rsi, %rax
+	ret
+
+  Function f2 in eBPF may look like::
+
+    f2:
+	bpf_mov R2, R1
+	bpf_add R1, 1
+	bpf_call f3
+	bpf_exit
+
+  If f2 is JITed and the pointer stored to ``_f2``. The calls f1 -> f2 -> f3 and
+  returns will be seamless. Without JIT, __bpf_prog_run() interpreter needs to
+  be used to call into f2.
+
+  For practical reasons all eBPF programs have only one argument 'ctx' which is
+  already placed into R1 (e.g. on __bpf_prog_run() startup) and the programs
+  can call kernel functions with up to 5 arguments. Calls with 6 or more arguments
+  are currently not supported, but these restrictions can be lifted if necessary
+  in the future.
+
+  On 64-bit architectures all register map to HW registers one to one. For
+  example, x86_64 JIT compiler can map them as ...
+
+  ::
+
+    R0 - rax
+    R1 - rdi
+    R2 - rsi
+    R3 - rdx
+    R4 - rcx
+    R5 - r8
+    R6 - rbx
+    R7 - r13
+    R8 - r14
+    R9 - r15
+    R10 - rbp
+
+  ... since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing
+  and rbx, r12 - r15 are callee saved.
+
+  Then the following eBPF pseudo-program::
+
+    bpf_mov R6, R1 /* save ctx */
+    bpf_mov R2, 2
+    bpf_mov R3, 3
+    bpf_mov R4, 4
+    bpf_mov R5, 5
+    bpf_call foo
+    bpf_mov R7, R0 /* save foo() return value */
+    bpf_mov R1, R6 /* restore ctx for next call */
+    bpf_mov R2, 6
+    bpf_mov R3, 7
+    bpf_mov R4, 8
+    bpf_mov R5, 9
+    bpf_call bar
+    bpf_add R0, R7
+    bpf_exit
+
+  After JIT to x86_64 may look like::
+
+    push %rbp
+    mov %rsp,%rbp
+    sub $0x228,%rsp
+    mov %rbx,-0x228(%rbp)
+    mov %r13,-0x220(%rbp)
+    mov %rdi,%rbx
+    mov $0x2,%esi
+    mov $0x3,%edx
+    mov $0x4,%ecx
+    mov $0x5,%r8d
+    callq foo
+    mov %rax,%r13
+    mov %rbx,%rdi
+    mov $0x6,%esi
+    mov $0x7,%edx
+    mov $0x8,%ecx
+    mov $0x9,%r8d
+    callq bar
+    add %r13,%rax
+    mov -0x228(%rbp),%rbx
+    mov -0x220(%rbp),%r13
+    leaveq
+    retq
+
+  Which is in this example equivalent in C to::
+
+    u64 bpf_filter(u64 ctx)
+    {
+	return foo(ctx, 2, 3, 4, 5) + bar(ctx, 6, 7, 8, 9);
+    }
+
+  In-kernel functions foo() and bar() with prototype: u64 (*)(u64 arg1, u64
+  arg2, u64 arg3, u64 arg4, u64 arg5); will receive arguments in proper
+  registers and place their return value into ``%rax`` which is R0 in eBPF.
+  Prologue and epilogue are emitted by JIT and are implicit in the
+  interpreter. R0-R5 are scratch registers, so eBPF program needs to preserve
+  them across the calls as defined by calling convention.
+
+  For example the following program is invalid::
+
+    bpf_mov R1, 1
+    bpf_call foo
+    bpf_mov R0, R1
+    bpf_exit
+
+  After the call the registers R1-R5 contain junk values and cannot be read.
+  An in-kernel verifier.rst is used to validate eBPF programs.
+
+Also in the new design, eBPF is limited to 4096 insns, which means that any
+program will terminate quickly and will only call a fixed number of kernel
+functions. Original BPF and eBPF are two operand instructions,
+which helps to do one-to-one mapping between eBPF insn and x86 insn during JIT.
+
+The input context pointer for invoking the interpreter function is generic,
+its content is defined by a specific use case. For seccomp register R1 points
+to seccomp_data, for converted BPF filters R1 points to a skb.
+
+A program, that is translated internally consists of the following elements::
+
+  op:16, jt:8, jf:8, k:32    ==>    op:8, dst_reg:4, src_reg:4, off:16, imm:32
+
+So far 87 eBPF instructions were implemented. 8-bit 'op' opcode field
+has room for new instructions. Some of them may use 16/24/32 byte encoding. New
+instructions must be multiple of 8 bytes to preserve backward compatibility.
+
+eBPF is a general purpose RISC instruction set. Not every register and
+every instruction are used during translation from original BPF to eBPF.
+For example, socket filters are not using ``exclusive add`` instruction, but
+tracing filters may do to maintain counters of events, for example. Register R9
+is not used by socket filters either, but more complex filters may be running
+out of registers and would have to resort to spill/fill to stack.
+
+eBPF can be used as a generic assembler for last step performance
+optimizations, socket filters and seccomp are using it as assembler. Tracing
+filters may use it as assembler to generate code from kernel. In kernel usage
+may not be bounded by security considerations, since generated eBPF code
+may be optimizing internal code path and not being exposed to the user space.
+Safety of eBPF can come from the verifier.rst. In such use cases as
+described, it may be used as safe instruction set.
+
+Just like the original BPF, eBPF runs within a controlled environment,
+is deterministic and the kernel can easily prove that. The safety of the program
+can be determined in two steps: first step does depth-first-search to disallow
+loops and other CFG validation; second step starts from the first insn and
+descends all possible paths. It simulates execution of every insn and observes
+the state change of registers and stack.
+
+opcode encoding
+===============
+
+eBPF is reusing most of the opcode encoding from classic to simplify conversion
+of classic BPF to eBPF.
+
+For arithmetic and jump instructions the 8-bit 'code' field is divided into three
+parts::
+
+  +----------------+--------+--------------------+
+  |   4 bits       |  1 bit |   3 bits           |
+  | operation code | source | instruction class  |
+  +----------------+--------+--------------------+
+  (MSB)                                      (LSB)
+
+Three LSB bits store instruction class which is one of:
+
+  ===================     ===============
+  Classic BPF classes     eBPF classes
+  ===================     ===============
+  BPF_LD    0x00          BPF_LD    0x00
+  BPF_LDX   0x01          BPF_LDX   0x01
+  BPF_ST    0x02          BPF_ST    0x02
+  BPF_STX   0x03          BPF_STX   0x03
+  BPF_ALU   0x04          BPF_ALU   0x04
+  BPF_JMP   0x05          BPF_JMP   0x05
+  BPF_RET   0x06          BPF_JMP32 0x06
+  BPF_MISC  0x07          BPF_ALU64 0x07
+  ===================     ===============
+
+The 4th bit encodes the source operand ...
+
+    ::
+
+	BPF_K     0x00
+	BPF_X     0x08
+
+ * in classic BPF, this means::
+
+	BPF_SRC(code) == BPF_X - use register X as source operand
+	BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
+
+ * in eBPF, this means::
+
+	BPF_SRC(code) == BPF_X - use 'src_reg' register as source operand
+	BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
+
+... and four MSB bits store operation code.
+
+If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of::
+
+  BPF_ADD   0x00
+  BPF_SUB   0x10
+  BPF_MUL   0x20
+  BPF_DIV   0x30
+  BPF_OR    0x40
+  BPF_AND   0x50
+  BPF_LSH   0x60
+  BPF_RSH   0x70
+  BPF_NEG   0x80
+  BPF_MOD   0x90
+  BPF_XOR   0xa0
+  BPF_MOV   0xb0  /* eBPF only: mov reg to reg */
+  BPF_ARSH  0xc0  /* eBPF only: sign extending shift right */
+  BPF_END   0xd0  /* eBPF only: endianness conversion */
+
+If BPF_CLASS(code) == BPF_JMP or BPF_JMP32 [ in eBPF ], BPF_OP(code) is one of::
+
+  BPF_JA    0x00  /* BPF_JMP only */
+  BPF_JEQ   0x10
+  BPF_JGT   0x20
+  BPF_JGE   0x30
+  BPF_JSET  0x40
+  BPF_JNE   0x50  /* eBPF only: jump != */
+  BPF_JSGT  0x60  /* eBPF only: signed '>' */
+  BPF_JSGE  0x70  /* eBPF only: signed '>=' */
+  BPF_CALL  0x80  /* eBPF BPF_JMP only: function call */
+  BPF_EXIT  0x90  /* eBPF BPF_JMP only: function return */
+  BPF_JLT   0xa0  /* eBPF only: unsigned '<' */
+  BPF_JLE   0xb0  /* eBPF only: unsigned '<=' */
+  BPF_JSLT  0xc0  /* eBPF only: signed '<' */
+  BPF_JSLE  0xd0  /* eBPF only: signed '<=' */
+
+So BPF_ADD | BPF_X | BPF_ALU means 32-bit addition in both classic BPF
+and eBPF. There are only two registers in classic BPF, so it means A += X.
+In eBPF it means dst_reg = (u32) dst_reg + (u32) src_reg; similarly,
+BPF_XOR | BPF_K | BPF_ALU means A ^= imm32 in classic BPF and analogous
+src_reg = (u32) src_reg ^ (u32) imm32 in eBPF.
+
+Classic BPF is using BPF_MISC class to represent A = X and X = A moves.
+eBPF is using BPF_MOV | BPF_X | BPF_ALU code instead. Since there are no
+BPF_MISC operations in eBPF, the class 7 is used as BPF_ALU64 to mean
+exactly the same operations as BPF_ALU, but with 64-bit wide operands
+instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition, i.e.:
+dst_reg = dst_reg + src_reg
+
+Classic BPF wastes the whole BPF_RET class to represent a single ``ret``
+operation. Classic BPF_RET | BPF_K means copy imm32 into return register
+and perform function exit. eBPF is modeled to match CPU, so BPF_JMP | BPF_EXIT
+in eBPF means function exit only. The eBPF program needs to store return
+value into register R0 before doing a BPF_EXIT. Class 6 in eBPF is used as
+BPF_JMP32 to mean exactly the same operations as BPF_JMP, but with 32-bit wide
+operands for the comparisons instead.
+
+For load and store instructions the 8-bit 'code' field is divided as::
+
+  +--------+--------+-------------------+
+  | 3 bits | 2 bits |   3 bits          |
+  |  mode  |  size  | instruction class |
+  +--------+--------+-------------------+
+  (MSB)                             (LSB)
+
+Size modifier is one of ...
+
+::
+
+  BPF_W   0x00    /* word */
+  BPF_H   0x08    /* half word */
+  BPF_B   0x10    /* byte */
+  BPF_DW  0x18    /* eBPF only, double word */
+
+... which encodes size of load/store operation::
+
+ B  - 1 byte
+ H  - 2 byte
+ W  - 4 byte
+ DW - 8 byte (eBPF only)
+
+Mode modifier is one of::
+
+  BPF_IMM     0x00  /* used for 32-bit mov in classic BPF and 64-bit in eBPF */
+  BPF_ABS     0x20
+  BPF_IND     0x40
+  BPF_MEM     0x60
+  BPF_LEN     0x80  /* classic BPF only, reserved in eBPF */
+  BPF_MSH     0xa0  /* classic BPF only, reserved in eBPF */
+  BPF_ATOMIC  0xc0  /* eBPF only, atomic operations */
diff --git a/Documentation/bpf/index.rst b/Documentation/bpf/index.rst
index 91ba5a62026ba..ef5c996547ecb 100644
--- a/Documentation/bpf/index.rst
+++ b/Documentation/bpf/index.rst
@@ -21,6 +21,7 @@ that goes into great technical depth about the BPF Architecture.
    helpers
    programs
    maps
+   classic_vs_extended.rst
    bpf_licensing
    test_debug
    other
diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index fa469078301be..3967842e00234 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -3,253 +3,27 @@
 eBPF Instruction Set
 ====================
 
-eBPF is designed to be JITed with one to one mapping, which can also open up
-the possibility for GCC/LLVM compilers to generate optimized eBPF code through
-an eBPF backend that performs almost as fast as natively compiled code.
-
-Some core changes of the eBPF format from classic BPF:
-
-- Number of registers increase from 2 to 10:
-
-  The old format had two registers A and X, and a hidden frame pointer. The
-  new layout extends this to be 10 internal registers and a read-only frame
-  pointer. Since 64-bit CPUs are passing arguments to functions via registers
-  the number of args from eBPF program to in-kernel function is restricted
-  to 5 and one register is used to accept return value from an in-kernel
-  function. Natively, x86_64 passes first 6 arguments in registers, aarch64/
-  sparcv9/mips64 have 7 - 8 registers for arguments; x86_64 has 6 callee saved
-  registers, and aarch64/sparcv9/mips64 have 11 or more callee saved registers.
-
-  Therefore, eBPF calling convention is defined as:
-
-    * R0	- return value from in-kernel function, and exit value for eBPF program
-    * R1 - R5	- arguments from eBPF program to in-kernel function
-    * R6 - R9	- callee saved registers that in-kernel function will preserve
-    * R10	- read-only frame pointer to access stack
-
-  Thus, all eBPF registers map one to one to HW registers on x86_64, aarch64,
-  etc, and eBPF calling convention maps directly to ABIs used by the kernel on
-  64-bit architectures.
-
-  On 32-bit architectures JIT may map programs that use only 32-bit arithmetic
-  and may let more complex programs to be interpreted.
-
-  R0 - R5 are scratch registers and eBPF program needs spill/fill them if
-  necessary across calls. Note that there is only one eBPF program (== one
-  eBPF main routine) and it cannot call other eBPF functions, it can only
-  call predefined in-kernel functions, though.
-
-- Register width increases from 32-bit to 64-bit:
-
-  Still, the semantics of the original 32-bit ALU operations are preserved
-  via 32-bit subregisters. All eBPF registers are 64-bit with 32-bit lower
-  subregisters that zero-extend into 64-bit if they are being written to.
-  That behavior maps directly to x86_64 and arm64 subregister definition, but
-  makes other JITs more difficult.
-
-  32-bit architectures run 64-bit eBPF programs via interpreter.
-  Their JITs may convert BPF programs that only use 32-bit subregisters into
-  native instruction set and let the rest being interpreted.
-
-  Operation is 64-bit, because on 64-bit architectures, pointers are also
-  64-bit wide, and we want to pass 64-bit values in/out of kernel functions,
-  so 32-bit eBPF registers would otherwise require to define register-pair
-  ABI, thus, there won't be able to use a direct eBPF register to HW register
-  mapping and JIT would need to do combine/split/move operations for every
-  register in and out of the function, which is complex, bug prone and slow.
-  Another reason is the use of atomic 64-bit counters.
-
-- Conditional jt/jf targets replaced with jt/fall-through:
-
-  While the original design has constructs such as ``if (cond) jump_true;
-  else jump_false;``, they are being replaced into alternative constructs like
-  ``if (cond) jump_true; /* else fall-through */``.
-
-- Introduces bpf_call insn and register passing convention for zero overhead
-  calls from/to other kernel functions:
-
-  Before an in-kernel function call, the eBPF program needs to
-  place function arguments into R1 to R5 registers to satisfy calling
-  convention, then the interpreter will take them from registers and pass
-  to in-kernel function. If R1 - R5 registers are mapped to CPU registers
-  that are used for argument passing on given architecture, the JIT compiler
-  doesn't need to emit extra moves. Function arguments will be in the correct
-  registers and BPF_CALL instruction will be JITed as single 'call' HW
-  instruction. This calling convention was picked to cover common call
-  situations without performance penalty.
-
-  After an in-kernel function call, R1 - R5 are reset to unreadable and R0 has
-  a return value of the function. Since R6 - R9 are callee saved, their state
-  is preserved across the call.
-
-  For example, consider three C functions::
-
-    u64 f1() { return (*_f2)(1); }
-    u64 f2(u64 a) { return f3(a + 1, a); }
-    u64 f3(u64 a, u64 b) { return a - b; }
-
-  GCC can compile f1, f3 into x86_64::
-
-    f1:
-	movl $1, %edi
-	movq _f2(%rip), %rax
-	jmp  *%rax
-    f3:
-	movq %rdi, %rax
-	subq %rsi, %rax
-	ret
-
-  Function f2 in eBPF may look like::
-
-    f2:
-	bpf_mov R2, R1
-	bpf_add R1, 1
-	bpf_call f3
-	bpf_exit
-
-  If f2 is JITed and the pointer stored to ``_f2``. The calls f1 -> f2 -> f3 and
-  returns will be seamless. Without JIT, __bpf_prog_run() interpreter needs to
-  be used to call into f2.
-
-  For practical reasons all eBPF programs have only one argument 'ctx' which is
-  already placed into R1 (e.g. on __bpf_prog_run() startup) and the programs
-  can call kernel functions with up to 5 arguments. Calls with 6 or more arguments
-  are currently not supported, but these restrictions can be lifted if necessary
-  in the future.
-
-  On 64-bit architectures all register map to HW registers one to one. For
-  example, x86_64 JIT compiler can map them as ...
-
-  ::
-
-    R0 - rax
-    R1 - rdi
-    R2 - rsi
-    R3 - rdx
-    R4 - rcx
-    R5 - r8
-    R6 - rbx
-    R7 - r13
-    R8 - r14
-    R9 - r15
-    R10 - rbp
-
-  ... since x86_64 ABI mandates rdi, rsi, rdx, rcx, r8, r9 for argument passing
-  and rbx, r12 - r15 are callee saved.
-
-  Then the following eBPF pseudo-program::
-
-    bpf_mov R6, R1 /* save ctx */
-    bpf_mov R2, 2
-    bpf_mov R3, 3
-    bpf_mov R4, 4
-    bpf_mov R5, 5
-    bpf_call foo
-    bpf_mov R7, R0 /* save foo() return value */
-    bpf_mov R1, R6 /* restore ctx for next call */
-    bpf_mov R2, 6
-    bpf_mov R3, 7
-    bpf_mov R4, 8
-    bpf_mov R5, 9
-    bpf_call bar
-    bpf_add R0, R7
-    bpf_exit
-
-  After JIT to x86_64 may look like::
-
-    push %rbp
-    mov %rsp,%rbp
-    sub $0x228,%rsp
-    mov %rbx,-0x228(%rbp)
-    mov %r13,-0x220(%rbp)
-    mov %rdi,%rbx
-    mov $0x2,%esi
-    mov $0x3,%edx
-    mov $0x4,%ecx
-    mov $0x5,%r8d
-    callq foo
-    mov %rax,%r13
-    mov %rbx,%rdi
-    mov $0x6,%esi
-    mov $0x7,%edx
-    mov $0x8,%ecx
-    mov $0x9,%r8d
-    callq bar
-    add %r13,%rax
-    mov -0x228(%rbp),%rbx
-    mov -0x220(%rbp),%r13
-    leaveq
-    retq
-
-  Which is in this example equivalent in C to::
-
-    u64 bpf_filter(u64 ctx)
-    {
-	return foo(ctx, 2, 3, 4, 5) + bar(ctx, 6, 7, 8, 9);
-    }
-
-  In-kernel functions foo() and bar() with prototype: u64 (*)(u64 arg1, u64
-  arg2, u64 arg3, u64 arg4, u64 arg5); will receive arguments in proper
-  registers and place their return value into ``%rax`` which is R0 in eBPF.
-  Prologue and epilogue are emitted by JIT and are implicit in the
-  interpreter. R0-R5 are scratch registers, so eBPF program needs to preserve
-  them across the calls as defined by calling convention.
-
-  For example the following program is invalid::
-
-    bpf_mov R1, 1
-    bpf_call foo
-    bpf_mov R0, R1
-    bpf_exit
-
-  After the call the registers R1-R5 contain junk values and cannot be read.
-  An in-kernel verifier.rst is used to validate eBPF programs.
-
-Also in the new design, eBPF is limited to 4096 insns, which means that any
-program will terminate quickly and will only call a fixed number of kernel
-functions. Original BPF and eBPF are two operand instructions,
-which helps to do one-to-one mapping between eBPF insn and x86 insn during JIT.
-
-The input context pointer for invoking the interpreter function is generic,
-its content is defined by a specific use case. For seccomp register R1 points
-to seccomp_data, for converted BPF filters R1 points to a skb.
-
-A program, that is translated internally consists of the following elements::
-
-  op:16, jt:8, jf:8, k:32    ==>    op:8, dst_reg:4, src_reg:4, off:16, imm:32
-
-So far 87 eBPF instructions were implemented. 8-bit 'op' opcode field
-has room for new instructions. Some of them may use 16/24/32 byte encoding. New
-instructions must be multiple of 8 bytes to preserve backward compatibility.
-
-eBPF is a general purpose RISC instruction set. Not every register and
-every instruction are used during translation from original BPF to eBPF.
-For example, socket filters are not using ``exclusive add`` instruction, but
-tracing filters may do to maintain counters of events, for example. Register R9
-is not used by socket filters either, but more complex filters may be running
-out of registers and would have to resort to spill/fill to stack.
-
-eBPF can be used as a generic assembler for last step performance
-optimizations, socket filters and seccomp are using it as assembler. Tracing
-filters may use it as assembler to generate code from kernel. In kernel usage
-may not be bounded by security considerations, since generated eBPF code
-may be optimizing internal code path and not being exposed to the user space.
-Safety of eBPF can come from the verifier.rst. In such use cases as
-described, it may be used as safe instruction set.
-
-Just like the original BPF, eBPF runs within a controlled environment,
-is deterministic and the kernel can easily prove that. The safety of the program
-can be determined in two steps: first step does depth-first-search to disallow
-loops and other CFG validation; second step starts from the first insn and
-descends all possible paths. It simulates execution of every insn and observes
-the state change of registers and stack.
+Registers and calling convention
+================================
+
+eBPF has 10 general purpose registers and a read-only frame pointer register,
+all of which are 64-bits wide.
+
+The eBPF calling convention is defined as:
+
+ * R0: return value from function calls, and exit value for eBPF programs
+ * R1 - R5: arguments for function calls
+ * R6 - R9: callee saved registers that function calls will preserve
+ * R10: read-only frame pointer to access stack
+
+R0 - R5 are scratch registers and eBPF programs needs to spill/fill them if
+necessary across calls.
 
 eBPF opcode encoding
 ====================
 
-eBPF is reusing most of the opcode encoding from classic to simplify conversion
-of classic BPF to eBPF. For arithmetic and jump instructions the 8-bit 'code'
-field is divided into three parts::
+For arithmetic and jump instructions the 8-bit 'opcode' field is divided into
+three parts::
 
   +----------------+--------+--------------------+
   |   4 bits       |  1 bit |   3 bits           |
@@ -259,39 +33,29 @@ field is divided into three parts::
 
 Three LSB bits store instruction class which is one of:
 
-  ===================     ===============
-  Classic BPF classes     eBPF classes
-  ===================     ===============
-  BPF_LD    0x00          BPF_LD    0x00
-  BPF_LDX   0x01          BPF_LDX   0x01
-  BPF_ST    0x02          BPF_ST    0x02
-  BPF_STX   0x03          BPF_STX   0x03
-  BPF_ALU   0x04          BPF_ALU   0x04
-  BPF_JMP   0x05          BPF_JMP   0x05
-  BPF_RET   0x06          BPF_JMP32 0x06
-  BPF_MISC  0x07          BPF_ALU64 0x07
-  ===================     ===============
+  ========= =====
+  class     value
+  ========= =====
+  BPF_LD    0x00
+  BPF_LDX   0x01
+  BPF_ST    0x02
+  BPF_STX   0x03
+  BPF_ALU   0x04
+  BPF_JMP   0x05
+  BPF_JMP32 0x06
+  BPF_ALU64 0x07
+  ========= =====
 
 When BPF_CLASS(code) == BPF_ALU or BPF_JMP, 4th bit encodes source operand ...
 
-    ::
-
-	BPF_K     0x00
-	BPF_X     0x08
-
- * in classic BPF, this means::
-
-	BPF_SRC(code) == BPF_X - use register X as source operand
-	BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
-
- * in eBPF, this means::
+::
 
-	BPF_SRC(code) == BPF_X - use 'src_reg' register as source operand
-	BPF_SRC(code) == BPF_K - use 32-bit immediate as source operand
+  BPF_K     0x00 /* use 32-bit immediate as source operand */
+  BPF_X     0x08 /* use 'src_reg' register as source operand */
 
 ... and four MSB bits store operation code.
 
-If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of::
+If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 BPF_OP(code) is one of::
 
   BPF_ADD   0x00
   BPF_SUB   0x10
@@ -304,45 +68,43 @@ If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 [ in eBPF ], BPF_OP(code) is one of::
   BPF_NEG   0x80
   BPF_MOD   0x90
   BPF_XOR   0xa0
-  BPF_MOV   0xb0  /* eBPF only: mov reg to reg */
-  BPF_ARSH  0xc0  /* eBPF only: sign extending shift right */
-  BPF_END   0xd0  /* eBPF only: endianness conversion */
+  BPF_MOV   0xb0  /* mov reg to reg */
+  BPF_ARSH  0xc0  /* sign extending shift right */
+  BPF_END   0xd0  /* endianness conversion */
 
-If BPF_CLASS(code) == BPF_JMP or BPF_JMP32 [ in eBPF ], BPF_OP(code) is one of::
+If BPF_CLASS(code) == BPF_JMP or BPF_JMP32 BPF_OP(code) is one of::
 
   BPF_JA    0x00  /* BPF_JMP only */
   BPF_JEQ   0x10
   BPF_JGT   0x20
   BPF_JGE   0x30
   BPF_JSET  0x40
-  BPF_JNE   0x50  /* eBPF only: jump != */
-  BPF_JSGT  0x60  /* eBPF only: signed '>' */
-  BPF_JSGE  0x70  /* eBPF only: signed '>=' */
-  BPF_CALL  0x80  /* eBPF BPF_JMP only: function call */
-  BPF_EXIT  0x90  /* eBPF BPF_JMP only: function return */
-  BPF_JLT   0xa0  /* eBPF only: unsigned '<' */
-  BPF_JLE   0xb0  /* eBPF only: unsigned '<=' */
-  BPF_JSLT  0xc0  /* eBPF only: signed '<' */
-  BPF_JSLE  0xd0  /* eBPF only: signed '<=' */
-
-So BPF_ADD | BPF_X | BPF_ALU means 32-bit addition in both classic BPF
-and eBPF. There are only two registers in classic BPF, so it means A += X.
-In eBPF it means dst_reg = (u32) dst_reg + (u32) src_reg; similarly,
-BPF_XOR | BPF_K | BPF_ALU means A ^= imm32 in classic BPF and analogous
-src_reg = (u32) src_reg ^ (u32) imm32 in eBPF.
-
-Classic BPF is using BPF_MISC class to represent A = X and X = A moves.
-eBPF is using BPF_MOV | BPF_X | BPF_ALU code instead. Since there are no
-BPF_MISC operations in eBPF, the class 7 is used as BPF_ALU64 to mean
-exactly the same operations as BPF_ALU, but with 64-bit wide operands
-instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition, i.e.:
-dst_reg = dst_reg + src_reg
-
-Classic BPF wastes the whole BPF_RET class to represent a single ``ret``
-operation. Classic BPF_RET | BPF_K means copy imm32 into return register
-and perform function exit. eBPF is modeled to match CPU, so BPF_JMP | BPF_EXIT
-in eBPF means function exit only. The eBPF program needs to store return
-value into register R0 before doing a BPF_EXIT. Class 6 in eBPF is used as
+  BPF_JNE   0x50  /* jump != */
+  BPF_JSGT  0x60  /* signed '>' */
+  BPF_JSGE  0x70  /* signed '>=' */
+  BPF_CALL  0x80  /* function call */
+  BPF_EXIT  0x90  /*  function return */
+  BPF_JLT   0xa0  /* unsigned '<' */
+  BPF_JLE   0xb0  /* unsigned '<=' */
+  BPF_JSLT  0xc0  /* signed '<' */
+  BPF_JSLE  0xd0  /* signed '<=' */
+
+So BPF_ADD | BPF_X | BPF_ALU means::
+
+  dst_reg = (u32) dst_reg + (u32) src_reg;
+
+Similarly, BPF_XOR | BPF_K | BPF_ALU means::
+
+  src_reg = (u32) src_reg ^ (u32) imm32
+
+eBPF is using BPF_MOV | BPF_X | BPF_ALU to represent A = B moves.  BPF_ALU64
+is used to mean exactly the same operations as BPF_ALU, but with 64-bit wide
+operands instead. So BPF_ADD | BPF_X | BPF_ALU64 means 64-bit addition, i.e.::
+
+  dst_reg = dst_reg + src_reg
+
+BPF_JMP | BPF_EXIT means function exit only. The eBPF program needs to store
+the return value into register R0 before doing a BPF_EXIT. Class 6 is used as
 BPF_JMP32 to mean exactly the same operations as BPF_JMP, but with 32-bit wide
 operands for the comparisons instead.
 
@@ -361,29 +123,27 @@ Size modifier is one of ...
   BPF_W   0x00    /* word */
   BPF_H   0x08    /* half word */
   BPF_B   0x10    /* byte */
-  BPF_DW  0x18    /* eBPF only, double word */
+  BPF_DW  0x18    /* double word */
 
 ... which encodes size of load/store operation::
 
  B  - 1 byte
  H  - 2 byte
  W  - 4 byte
- DW - 8 byte (eBPF only)
+ DW - 8 byte
 
 Mode modifier is one of::
 
-  BPF_IMM     0x00  /* used for 32-bit mov in classic BPF and 64-bit in eBPF */
+  BPF_IMM     0x00  /* used for 64-bit mov */
   BPF_ABS     0x20
   BPF_IND     0x40
   BPF_MEM     0x60
-  BPF_LEN     0x80  /* classic BPF only, reserved in eBPF */
-  BPF_MSH     0xa0  /* classic BPF only, reserved in eBPF */
-  BPF_ATOMIC  0xc0  /* eBPF only, atomic operations */
+  BPF_ATOMIC  0xc0  /* atomic operations */
 
 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 to have strong performance of
+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
@@ -405,7 +165,7 @@ For example::
     R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32))
     and R1 - R5 were scratched.
 
-Unlike classic BPF instruction set, eBPF has generic load/store operations::
+eBPF has generic load/store operations::
 
     BPF_MEM | <size> | BPF_STX:  *(size *) (dst_reg + off) = src_reg
     BPF_MEM | <size> | BPF_ST:   *(size *) (dst_reg + off) = imm32
@@ -460,5 +220,3 @@ 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.
-Classic BPF has similar instruction: ``BPF_LD | BPF_W | BPF_IMM`` which loads
-32-bit immediate value into a register.
-- 
2.30.2

[PATCH 3/4] bpf, docs: Generate nicer tables for instruction encodings

[Christoph Hellwig]

Use RST tables that are nicely readable both in plain ascii as well as
in html to render the instruction encodings, and add a few subheadings
to better structure the text.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
 Documentation/bpf/instruction-set.rst | 158 ++++++++++++++++----------
 1 file changed, 95 insertions(+), 63 deletions(-)

diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index 3967842e00234..4e3041cf04325 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -19,19 +19,10 @@ The eBPF calling convention is defined as:
 R0 - R5 are scratch registers and eBPF programs needs to spill/fill them if
 necessary across calls.
 
-eBPF opcode encoding
-====================
-
-For arithmetic and jump instructions the 8-bit 'opcode' field is divided into
-three parts::
-
-  +----------------+--------+--------------------+
-  |   4 bits       |  1 bit |   3 bits           |
-  | operation code | source | instruction class  |
-  +----------------+--------+--------------------+
-  (MSB)                                      (LSB)
+Instruction classes
+===================
 
-Three LSB bits store instruction class which is one of:
+The three LSB bits of the 'opcode' field store the instruction class:
 
   ========= =====
   class     value
@@ -46,17 +37,34 @@ Three LSB bits store instruction class which is one of:
   BPF_ALU64 0x07
   ========= =====
 
-When BPF_CLASS(code) == BPF_ALU or BPF_JMP, 4th bit encodes source operand ...
+Arithmetic and jump instructions
+================================
+
+For arithmetic and jump instructions (BPF_ALU, BPF_ALU64, BPF_JMP and
+BPF_JMP32), the 8-bit 'opcode' field is divided into three parts:
 
-::
+  ==============  ======  =================
+  4 bits (MSB)    1 bit   3 bits (LSB)
+  ==============  ======  =================
+  operation code  source  instruction class
+  ==============  ======  =================
 
-  BPF_K     0x00 /* use 32-bit immediate as source operand */
-  BPF_X     0x08 /* use 'src_reg' register as source operand */
+The 4th bit encodes the source operand:
 
-... and four MSB bits store operation code.
+  ======  =====  ========================================
+  source  value  description
+  ======  =====  ========================================
+  BPF_K   0x00   use 32-bit immediate as source operand
+  BPF_X   0x08   use 'src_reg' register as source operand
+  ======  =====  ========================================
 
-If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 BPF_OP(code) is one of::
+The four MSB bits store the operation code.
 
+For class BPF_ALU or BPF_ALU64:
+
+  ========  =====  =========================
+  code      value  description
+  ========  =====  =========================
   BPF_ADD   0x00
   BPF_SUB   0x10
   BPF_MUL   0x20
@@ -68,26 +76,31 @@ If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 BPF_OP(code) is one of::
   BPF_NEG   0x80
   BPF_MOD   0x90
   BPF_XOR   0xa0
-  BPF_MOV   0xb0  /* mov reg to reg */
-  BPF_ARSH  0xc0  /* sign extending shift right */
-  BPF_END   0xd0  /* endianness conversion */
+  BPF_MOV   0xb0   mov reg to reg
+  BPF_ARSH  0xc0   sign extending shift right
+  BPF_END   0xd0   endianness conversion
+  ========  =====  =========================
 
-If BPF_CLASS(code) == BPF_JMP or BPF_JMP32 BPF_OP(code) is one of::
+For class BPF_JMP or BPF_JMP32:
 
-  BPF_JA    0x00  /* BPF_JMP only */
+  ========  =====  =========================
+  code      value  description
+  ========  =====  =========================
+  BPF_JA    0x00   BPF_JMP only
   BPF_JEQ   0x10
   BPF_JGT   0x20
   BPF_JGE   0x30
   BPF_JSET  0x40
-  BPF_JNE   0x50  /* jump != */
-  BPF_JSGT  0x60  /* signed '>' */
-  BPF_JSGE  0x70  /* signed '>=' */
-  BPF_CALL  0x80  /* function call */
-  BPF_EXIT  0x90  /*  function return */
-  BPF_JLT   0xa0  /* unsigned '<' */
-  BPF_JLE   0xb0  /* unsigned '<=' */
-  BPF_JSLT  0xc0  /* signed '<' */
-  BPF_JSLE  0xd0  /* signed '<=' */
+  BPF_JNE   0x50   jump '!='
+  BPF_JSGT  0x60   signed '>'
+  BPF_JSGE  0x70   signed '>='
+  BPF_CALL  0x80   function call
+  BPF_EXIT  0x90   function return
+  BPF_JLT   0xa0   unsigned '<'
+  BPF_JLE   0xb0   unsigned '<='
+  BPF_JSLT  0xc0   signed '<'
+  BPF_JSLE  0xd0   signed '<='
+  ========  =====  =========================
 
 So BPF_ADD | BPF_X | BPF_ALU means::
 
@@ -108,37 +121,58 @@ the return value into register R0 before doing a BPF_EXIT. Class 6 is used as
 BPF_JMP32 to mean exactly the same operations as BPF_JMP, but with 32-bit wide
 operands for the comparisons instead.
 
-For load and store instructions the 8-bit 'code' field is divided as::
 
-  +--------+--------+-------------------+
-  | 3 bits | 2 bits |   3 bits          |
-  |  mode  |  size  | instruction class |
-  +--------+--------+-------------------+
-  (MSB)                             (LSB)
+Load and store instructions
+===========================
+
+For load and store instructions (BPF_LD, BPF_LDX, BPF_ST and BPF_STX), the
+8-bit 'opcode' field is divided as:
+
+  ============  ======  =================
+  3 bits (MSB)  2 bits  3 bits (LSB)
+  ============  ======  =================
+  mode          size    instruction class
+  ============  ======  =================
+
+The size modifier is one of:
 
-Size modifier is one of ...
+  =============  =====  =====================
+  size modifier  value  description
+  =============  =====  =====================
+  BPF_W          0x00   word        (4 bytes)
+  BPF_H          0x08   half word   (2 bytes)
+  BPF_B          0x10   byte
+  BPF_DW         0x18   double word (8 bytes)
+  =============  =====  =====================
 
-::
+The mode modifier is one of:
 
-  BPF_W   0x00    /* word */
-  BPF_H   0x08    /* half word */
-  BPF_B   0x10    /* byte */
-  BPF_DW  0x18    /* double word */
+  =============  =====  =====================
+  mode modifier  value  description
+  =============  =====  =====================
+  BPF_IMM        0x00   used for 64-bit mov
+  BPF_ABS        0x20
+  BPF_IND        0x40
+  BPF_MEM        0x60
+  BPF_ATOMIC     0xc0   atomic operations 
+  =============  =====  =====================
 
-... which encodes size of load/store operation::
+BPF_MEM | <size> | BPF_STX means::
 
- B  - 1 byte
- H  - 2 byte
- W  - 4 byte
- DW - 8 byte
+  *(size *) (dst_reg + off) = src_reg
 
-Mode modifier is one of::
+BPF_MEM | <size> | BPF_ST means::
 
-  BPF_IMM     0x00  /* used for 64-bit mov */
-  BPF_ABS     0x20
-  BPF_IND     0x40
-  BPF_MEM     0x60
-  BPF_ATOMIC  0xc0  /* atomic operations */
+  *(size *) (dst_reg + off) = imm32
+
+BPF_MEM | <size> | BPF_LDX means::
+
+  dst_reg = *(size *) (src_reg + off)
+
+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.
@@ -165,15 +199,10 @@ For example::
     R0 = ntohl(*(u32 *) (((struct sk_buff *) R6)->data + src_reg + imm32))
     and R1 - R5 were scratched.
 
-eBPF has generic load/store operations::
+Atomic operations
+-----------------
 
-    BPF_MEM | <size> | BPF_STX:  *(size *) (dst_reg + off) = src_reg
-    BPF_MEM | <size> | BPF_ST:   *(size *) (dst_reg + off) = imm32
-    BPF_MEM | <size> | BPF_LDX:  dst_reg = *(size *) (src_reg + off)
-
-Where size is one of: BPF_B or BPF_H or BPF_W or BPF_DW.
-
-It also includes atomic operations, which use the immediate field for extra
+eBPF includes atomic operations, which use the immediate field for extra
 encoding::
 
    .imm = BPF_ADD, .code = BPF_ATOMIC | BPF_W  | BPF_STX: lock xadd *(u32 *)(dst_reg + off16) += src_reg
@@ -217,6 +246,9 @@ You may encounter ``BPF_XADD`` - this is a legacy name for ``BPF_ATOMIC``,
 referring to the exclusive-add operation encoded when the immediate field is
 zero.
 
+16-byte instructions
+--------------------
+
 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.
-- 
2.30.2

Re: [PATCH 3/4] bpf, docs: Generate nicer tables for instruction encodings

[Alexei Starovoitov]

On Thu, Dec 23, 2021 at 11:19:05AM +0100, Christoph Hellwig wrote:
>  
> +For class BPF_ALU or BPF_ALU64:
> +
> +  ========  =====  =========================
> +  code      value  description
> +  ========  =====  =========================
>    BPF_ADD   0x00
>    BPF_SUB   0x10
>    BPF_MUL   0x20
> @@ -68,26 +76,31 @@ If BPF_CLASS(code) == BPF_ALU or BPF_ALU64 BPF_OP(code) is one of::
>    BPF_NEG   0x80
>    BPF_MOD   0x90
>    BPF_XOR   0xa0
> -  BPF_MOV   0xb0  /* mov reg to reg */
> -  BPF_ARSH  0xc0  /* sign extending shift right */
> -  BPF_END   0xd0  /* endianness conversion */
> +  BPF_MOV   0xb0   mov reg to reg
> +  BPF_ARSH  0xc0   sign extending shift right
> +  BPF_END   0xd0   endianness conversion
> +  ========  =====  =========================
>  
> -If BPF_CLASS(code) == BPF_JMP or BPF_JMP32 BPF_OP(code) is one of::
> +For class BPF_JMP or BPF_JMP32:
>  
> -  BPF_JA    0x00  /* BPF_JMP only */
> +  ========  =====  =========================
> +  code      value  description
> +  ========  =====  =========================
> +  BPF_JA    0x00   BPF_JMP only
>    BPF_JEQ   0x10
>    BPF_JGT   0x20
>    BPF_JGE   0x30
>    BPF_JSET  0x40

Not your fault, but the new table looks odd with
only some opcodes documented.
Same issue with BPF_ALU table.
In the past the documented opcodes were for eBPF only and
not documented in both, so it wasn't that bad.
At least there was a reason for discrepancy.
Now it just odd.
May be add a comment to all rows?

> -  BPF_JNE   0x50  /* jump != */
> -  BPF_JSGT  0x60  /* signed '>' */
> -  BPF_JSGE  0x70  /* signed '>=' */
> -  BPF_CALL  0x80  /* function call */
> -  BPF_EXIT  0x90  /*  function return */
> -  BPF_JLT   0xa0  /* unsigned '<' */
> -  BPF_JLE   0xb0  /* unsigned '<=' */
> -  BPF_JSLT  0xc0  /* signed '<' */
> -  BPF_JSLE  0xd0  /* signed '<=' */
> +  BPF_JNE   0x50   jump '!='
> +  BPF_JSGT  0x60   signed '>'
> +  BPF_JSGE  0x70   signed '>='
> +  BPF_CALL  0x80   function call
> +  BPF_EXIT  0x90   function return
> +  BPF_JLT   0xa0   unsigned '<'
> +  BPF_JLE   0xb0   unsigned '<='
> +  BPF_JSLT  0xc0   signed '<'
> +  BPF_JSLE  0xd0   signed '<='
> +  ========  =====  =========================
>  
>  So BPF_ADD | BPF_X | BPF_ALU means::
>  
> @@ -108,37 +121,58 @@ the return value into register R0 before doing a BPF_EXIT. Class 6 is used as
>  BPF_JMP32 to mean exactly the same operations as BPF_JMP, but with 32-bit wide
>  operands for the comparisons instead.
>  
> -For load and store instructions the 8-bit 'code' field is divided as::
>  
> -  +--------+--------+-------------------+
> -  | 3 bits | 2 bits |   3 bits          |
> -  |  mode  |  size  | instruction class |
> -  +--------+--------+-------------------+
> -  (MSB)                             (LSB)
> +Load and store instructions
> +===========================
> +
> +For load and store instructions (BPF_LD, BPF_LDX, BPF_ST and BPF_STX), the
> +8-bit 'opcode' field is divided as:
> +
> +  ============  ======  =================
> +  3 bits (MSB)  2 bits  3 bits (LSB)
> +  ============  ======  =================
> +  mode          size    instruction class
> +  ============  ======  =================
> +
> +The size modifier is one of:
>  
> -Size modifier is one of ...
> +  =============  =====  =====================
> +  size modifier  value  description
> +  =============  =====  =====================
> +  BPF_W          0x00   word        (4 bytes)
> +  BPF_H          0x08   half word   (2 bytes)
> +  BPF_B          0x10   byte
> +  BPF_DW         0x18   double word (8 bytes)
> +  =============  =====  =====================
>  
> -::
> +The mode modifier is one of:
>  
> -  BPF_W   0x00    /* word */
> -  BPF_H   0x08    /* half word */
> -  BPF_B   0x10    /* byte */
> -  BPF_DW  0x18    /* double word */
> +  =============  =====  =====================
> +  mode modifier  value  description
> +  =============  =====  =====================
> +  BPF_IMM        0x00   used for 64-bit mov
> +  BPF_ABS        0x20
> +  BPF_IND        0x40
> +  BPF_MEM        0x60

May be say here that ABS and IND are legacy for compat with classic only?
and MEM is the most common modifier for load/store?

> +  BPF_ATOMIC     0xc0   atomic operations 

I removed trailing space in above line.

And applied all patches to bpf-next. Thanks!

Re: [PATCH 3/4] bpf, docs: Generate nicer tables for instruction encodings

[Christoph Hellwig]

On Thu, Dec 30, 2021 at 04:43:24PM -0800, Alexei Starovoitov wrote:
> > +  ========  =====  =========================
> > +  code      value  description
> > +  ========  =====  =========================
> > +  BPF_JA    0x00   BPF_JMP only
> >    BPF_JEQ   0x10
> >    BPF_JGT   0x20
> >    BPF_JGE   0x30
> >    BPF_JSET  0x40
> 
> Not your fault, but the new table looks odd with
> only some opcodes documented.
> Same issue with BPF_ALU table.
> In the past the documented opcodes were for eBPF only and
> not documented in both, so it wasn't that bad.
> At least there was a reason for discrepancy.
> Now it just odd.
> May be add a comment to all rows?

Yes, having the description everywhere would be good.  But I'll have to
do research to actually figure out what should go in there for some.

> > +  =============  =====  =====================
> > +  mode modifier  value  description
> > +  =============  =====  =====================
> > +  BPF_IMM        0x00   used for 64-bit mov
> > +  BPF_ABS        0x20
> > +  BPF_IND        0x40
> > +  BPF_MEM        0x60
> 
> May be say here that ABS and IND are legacy for compat with classic only?
> and MEM is the most common modifier for load/store?

Sure.

[PATCH 4/4] bpf, docs: Move the packet access instructions last in instruction-set.rst

[Christoph Hellwig]

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>
---
 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