[PATCH bpf-next v4] bpf, docs: Add docs on extended 64-bit immediate instructions

[PATCH bpf-next v4] bpf, docs: Add docs on extended 64-bit immediate instructions

[Dave Thaler]

From: Dave Thaler <dthaler@microsoft.com>

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: Dave Thaler <dthaler@microsoft.com>
---
 Documentation/bpf/instruction-set.rst | 57 +++++++++++++++++++++++----
 Documentation/bpf/linux-notes.rst     | 22 +++++++++++
 2 files changed, 71 insertions(+), 8 deletions(-)

diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
index b4464058905..b3efa4b74ec 100644
--- a/Documentation/bpf/instruction-set.rst
+++ b/Documentation/bpf/instruction-set.rst
@@ -401,14 +401,55 @@ and loaded back to ``R0``.
 -----------------------------
 
 Instructions with the ``BPF_IMM`` 'mode' modifier use the wide instruction
-encoding for an extra imm64 value.
-
-There is currently only one such instruction.
-
-``BPF_LD | BPF_DW | BPF_IMM`` means::
-
-  dst = imm64
-
+encoding defined in `Instruction encoding`_, and use the 'src' field of the
+basic instruction to hold an opcode subtype.
+
+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:
+
+=========================  ======  ===  =========================================  ===========  ==============
+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 POSIX 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 POSIX 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
 -------------------------------------
diff --git a/Documentation/bpf/linux-notes.rst b/Documentation/bpf/linux-notes.rst
index f43b9c797bc..508d009d3be 100644
--- a/Documentation/bpf/linux-notes.rst
+++ b/Documentation/bpf/linux-notes.rst
@@ -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
 =====================================
 
-- 
2.33.4

Re: [Bpf] [PATCH bpf-next v4] bpf, docs: Add docs on extended 64-bit immediate instructions

[Alexei Starovoitov]

On Sat, Mar 25, 2023 at 10:49 PM Dave Thaler
<dthaler1968=40googlemail.com@dmarc.ietf.org> wrote:
>
> From: Dave Thaler <dthaler@microsoft.com>
>
> 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: Dave Thaler <dthaler@microsoft.com>
> ---
>  Documentation/bpf/instruction-set.rst | 57 +++++++++++++++++++++++----
>  Documentation/bpf/linux-notes.rst     | 22 +++++++++++
>  2 files changed, 71 insertions(+), 8 deletions(-)
>
> diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
> index b4464058905..b3efa4b74ec 100644
> --- a/Documentation/bpf/instruction-set.rst
> +++ b/Documentation/bpf/instruction-set.rst
> @@ -401,14 +401,55 @@ and loaded back to ``R0``.
>  -----------------------------
>
>  Instructions with the ``BPF_IMM`` 'mode' modifier use the wide instruction
> -encoding for an extra imm64 value.
> -
> -There is currently only one such instruction.
> -
> -``BPF_LD | BPF_DW | BPF_IMM`` means::
> -
> -  dst = imm64
> -
> +encoding defined in `Instruction encoding`_, and use the 'src' field of the
> +basic instruction to hold an opcode subtype.
> +
> +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:
> +
> +=========================  ======  ===  =========================================  ===========  ==============
> +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 POSIX file descriptor into an address of a map (see `Maps`_)

I removed the screaming "POSIX" word, since "file descriptor" is
descriptive enough.

> +* 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.

Reformatted these sections to fit 80 char.
And applied.

Re: [PATCH bpf-next v4] bpf, docs: Add docs on extended 64-bit immediate instructions

[patchwork-bot+netdevbpf]

Hello:

This patch was applied to bpf/bpf-next.git (master)
by Alexei Starovoitov <ast@kernel.org>:

On Sun, 26 Mar 2023 05:49:46 +0000 you wrote:
> From: Dave Thaler <dthaler@microsoft.com>
> 
> 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
> 
> [...]

Here is the summary with links:
  - [bpf-next,v4] bpf, docs: Add docs on extended 64-bit immediate instructions
    https://git.kernel.org/bpf/bpf-next/c/16b7c970cc81

You are awesome, thank you!
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