Commit 16b7c970 authored by Dave Thaler's avatar Dave Thaler Committed by Alexei Starovoitov
Browse files

bpf, docs: Add docs on extended 64-bit immediate instructions 



Add docs on extended 64-bit immediate instructions, including six instructions
previously undocumented.  Include a brief description of maps and variables,
as used by those instructions.

V1 -> V2: rebased on top of latest master

V2 -> V3: addressed comments from Alexei

V3 -> V4: addressed comments from David Vernet

Signed-off-by: default avatarDave Thaler <dthaler@microsoft.com>
Link: https://lore.kernel.org/r/20230326054946.2331-1-dthaler1968@googlemail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent 92b2e810

Documentation/bpf/instruction-set.rst

+50 −8
Original line number Diff line number Diff line
@@ -416,14 +416,56 @@ and loaded back to ``R0``. 
-----------------------------



Instructions with the ``BPF_IMM`` 'mode' modifier use the wide instruction

encoding for an extra imm64 value.

encoding defined in `Instruction encoding`_, and use the 'src' field of the

basic instruction to hold an opcode subtype.



There is currently only one such instruction.

The following table defines a set of ``BPF_IMM | BPF_DW | BPF_LD`` instructions

with opcode subtypes in the 'src' field, using new terms such as "map"

defined further below:



``BPF_LD | BPF_DW | BPF_IMM`` means::



  dst = imm64

=========================  ======  ===  =========================================  ===========  ==============

opcode construction        opcode  src  pseudocode                                 imm type     dst type

=========================  ======  ===  =========================================  ===========  ==============

BPF_IMM | BPF_DW | BPF_LD  0x18    0x0  dst = imm64                                integer      integer

BPF_IMM | BPF_DW | BPF_LD  0x18    0x1  dst = map_by_fd(imm)                       map fd       map

BPF_IMM | BPF_DW | BPF_LD  0x18    0x2  dst = map_val(map_by_fd(imm)) + next_imm   map fd       data pointer

BPF_IMM | BPF_DW | BPF_LD  0x18    0x3  dst = var_addr(imm)                        variable id  data pointer

BPF_IMM | BPF_DW | BPF_LD  0x18    0x4  dst = code_addr(imm)                       integer      code pointer

BPF_IMM | BPF_DW | BPF_LD  0x18    0x5  dst = map_by_idx(imm)                      map index    map

BPF_IMM | BPF_DW | BPF_LD  0x18    0x6  dst = map_val(map_by_idx(imm)) + next_imm  map index    data pointer

=========================  ======  ===  =========================================  ===========  ==============



where



* map_by_fd(imm) means to convert a 32-bit file descriptor into an address of a map (see `Maps`_)

* map_by_idx(imm) means to convert a 32-bit index into an address of a map

* map_val(map) gets the address of the first value in a given map

* var_addr(imm) gets the address of a platform variable (see `Platform Variables`_) with a given id

* code_addr(imm) gets the address of the instruction at a specified relative offset in number of (64-bit) instructions

* the 'imm type' can be used by disassemblers for display

* the 'dst type' can be used for verification and JIT compilation purposes



Maps

~~~~



Maps are shared memory regions accessible by eBPF programs on some platforms.

A map can have various semantics as defined in a separate document, and may or

may not have a single contiguous memory region, but the 'map_val(map)' is

currently only defined for maps that do have a single contiguous memory region.



Each map can have a file descriptor (fd) if supported by the platform, where

'map_by_fd(imm)' means to get the map with the specified file descriptor. Each

BPF program can also be defined to use a set of maps associated with the

program at load time, and 'map_by_idx(imm)' means to get the map with the given

index in the set associated with the BPF program containing the instruction.



Platform Variables

~~~~~~~~~~~~~~~~~~



Platform variables are memory regions, identified by integer ids, exposed by

the runtime and accessible by BPF programs on some platforms.  The

'var_addr(imm)' operation means to get the address of the memory region

identified by the given id.



Legacy BPF Packet access instructions

-------------------------------------

Documentation/bpf/linux-notes.rst

+22 −0
Original line number Diff line number Diff line
@@ -20,6 +20,28 @@ integer would be read from a specified register, is not currently supported 
by the verifier.  Any programs with this instruction will fail to load

until such support is added.



Maps

====



Linux only supports the 'map_val(map)' operation on array maps with a single element.



Linux uses an fd_array to store maps associated with a BPF program. Thus,

map_by_idx(imm) uses the fd at that index in the array.



Variables

=========



The following 64-bit immediate instruction specifies that a variable address,

which corresponds to some integer stored in the 'imm' field, should be loaded:



=========================  ======  ===  =========================================  ===========  ==============

opcode construction        opcode  src  pseudocode                                 imm type     dst type

=========================  ======  ===  =========================================  ===========  ==============

BPF_IMM | BPF_DW | BPF_LD  0x18    0x3  dst = var_addr(imm)                        variable id  data pointer

=========================  ======  ===  =========================================  ===========  ==============



On Linux, this integer is a BTF ID.



Legacy BPF Packet access instructions

=====================================