Re: 64-bit immediate instructions clarification

[Yonghong Song <yonghong.song@linux.dev>]

On 1/25/24 5:12 PM, dthaler1968@googlemail.com wrote:
> The spec defines:
>> As discussed below in `64-bit immediate instructions`_, a 64-bit immediate
>> instruction uses a 64-bit immediate value that is constructed as follows.
>> The 64 bits following the basic instruction contain a pseudo instruction
>> using the same format but with opcode, dst_reg, src_reg, and offset all set to zero,
>> and imm containing the high 32 bits of the immediate value.
> [...]
>> imm64 = (next_imm << 32) | imm
> The 64-bit immediate instructions section then says:
>> Instructions with the ``BPF_IMM`` 'mode' modifier use the wide instruction
>> encoding defined in `Instruction encoding`_, and use the 'src' field of the
>> basic instruction to hold an opcode subtype.
> Some instructions then nicely state how to use the full 64 bit immediate value, such as
>> BPF_IMM | BPF_DW | BPF_LD  0x18    0x0  dst = imm64                                integer      integer
>> 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    0x6  dst = map_val(map_by_idx(imm)) + next_imm  map index    data pointer
> Others don't:
>> BPF_IMM | BPF_DW | BPF_LD  0x18    0x1  dst = map_by_fd(imm)                       map fd       map
>> 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
> How is next_imm used in those four?  Must it be 0?  Or can it be anything and it's ignored?
> Or is it used for something?

The other four must have next_imm to be 0. No use of next_imm in thee four insns kindly implies this.
See uapi bpf.h for details (search BPF_PSEUDO_MAP_FD).

>
> Dave
>