bpf: add special internal-only MOV instruction to resolve per-CPU addrs

Add a new BPF instruction for resolving absolute addresses of per-CPU
data from their per-CPU offsets. This instruction is internal-only and
users are not allowed to use them directly. They will only be used for
internal inlining optimizations for now between BPF verifier and BPF JITs.

We use a special BPF_MOV | BPF_ALU64 | BPF_X form with insn->off field
set to BPF_ADDR_PERCPU = -1. I used negative offset value to distinguish
them from positive ones used by user-exposed instructions.

Such instruction performs a resolution of a per-CPU offset stored in
a register to a valid kernel address which can be dereferenced. It is
useful in any use case where absolute address of a per-CPU data has to
be resolved (e.g., in inlining bpf_map_lookup_elem()).

BPF disassembler is also taught to recognize them to support dumping
final BPF assembly code (non-JIT'ed version).

Add arch-specific way for BPF JITs to mark support for this instructions.

This patch also adds support for these instructions in x86-64 BPF JIT.

Signed-off-by: Andrii Nakryiko <[email protected]>
Acked-by: John Fastabend <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

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