bpf: Add helpers to describe the R0: R2 return register pair

LLVM 23 added support for returning a value in two registers for an
__int128, or a struct/union whose size is greater than 8 but not more than
16 bytes: such a value comes back in the R0:R2 register pair, with R2
holding the upper half. See LLVM patches [1] and [2].

Later patches teach the JIT, live register analysis and the verifier itself
about that convention. They need to answer the same question: does this
subprogram return its value in a register pair? Add bpf_ret_reg_pair() up
front so it can be used in subsequent patches. It is answered from a
per-subprogram flag that bpf_compute_subprog_ret_regs() derives once from
the BTF prototype.

jit_requested only says that the JIT is enabled, not that it succeeded:
bpf_fixup_call_args() falls back to the interpreter when bpf_jit_subprogs()
fails with anything other than -EFAULT. So jit_required is still set once
the pair is modelled, which turns that fallback into a load failure rather
than a silent divergence from what was verified.

  [1] https://github.com/llvm/llvm-project/pull/190894
  [2] https://github.com/llvm/llvm-project/pull/206876

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://patch.msgid.link/20260819055249.3295283-1-yonghong.song@linux.dev
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

0 files changed, 0 insertions, 0 deletions