Re: [Bpf] [PATCH bpf-next v3] bpf, docs: Add docs on extended 64-bit immediate instructions

[David Vernet <void@manifault.com>]

On Sat, Mar 25, 2023 at 10:43:05PM +0000, Dave Thaler 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 map objects, and variables,
> as used by those instructions.
> 
> ---
> V1 -> V2: rebased on top of latest master
> 
> V2 -> V3: addressed comments from Alexei
> 
> Signed-off-by: Dave Thaler <dthaler@microsoft.com>
> ---
>  Documentation/bpf/instruction-set.rst | 56 +++++++++++++++++++++++----
>  Documentation/bpf/linux-notes.rst     | 13 +++++++
>  2 files changed, 61 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/bpf/instruction-set.rst b/Documentation/bpf/instruction-set.rst
> index db8789e6969..2c8347d63e7 100644
> --- a/Documentation/bpf/instruction-set.rst
> +++ b/Documentation/bpf/instruction-set.rst
> @@ -385,14 +385,54 @@ 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 instructions are defined, and use additional concepts defined below:

nit: Perhaps this is a bit clearer? Wdyt?

The following opcode subtypes are defined for `BPF_IMM | BPF_DQ |
BPF_LD` instructions, with 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 object (see `Map objects`_)
> +* map_by_idx(imm) means to convert a 32-bit index into an address of a map object
> +* map_val(map) gets the address of the first value in a given map object
> +* 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
> +
> +Map objects
> +~~~~~~~~~~~
> +
> +Maps are shared memory regions accessible by eBPF programs on some platforms, where we use the term "map object"
> +to refer to an object containing the data and metadata (e.g., size) about the memory region.
> +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 object 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 956b0c86699..2d161467105 100644
> --- a/Documentation/bpf/linux-notes.rst
> +++ b/Documentation/bpf/linux-notes.rst
> @@ -12,6 +12,19 @@ Byte swap instructions
>  
>  ``BPF_FROM_LE`` and ``BPF_FROM_BE`` exist as aliases for ``BPF_TO_LE`` and ``BPF_TO_BE`` respectively.
>  
> +Map objects
> +===========
> +
> +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_index(index) uses the fd at that index in the array.
> +
> +Variables
> +=========
> +
> +Linux uses BTF ids to identify variables.

Not quite sure exactly what this means. Linux uses BTF ids to identify
_types_, right? This doesn't seem like something that needs to be
specified as Linux specific either, even if it's not yet supported
elsewhere. Certain legacy things such as Linux-specific helpers make
sense, but not sure about BTF ids.

> +
>  Legacy BPF Packet access instructions
>  =====================================
>  
> -- 
> 2.33.4
> 
> -- 
> Bpf mailing list
> Bpf@ietf.org
> https://www.ietf.org/mailman/listinfo/bpf