bpf: Wire up JIT support for 16-byte kfunc returns

LLVM 23 returns an __int128, or a struct/union larger than 8 bytes and no
larger than 16 bytes, in the BPF R0:R2 register pair. The previous patch
added the shared helpers describing that convention; wire up the JIT side
so that the second half of the return value actually lands in R2.

Placing the second return half into R2 is possible on any JIT, but it needs
architecture-specific JIT work. Rather than requiring every JIT to
implement it at once, add a bpf_jit_supports_kfunc_ret_reg_pair()
capability, defaulting to false in the generic core; an architecture opts
in once its JIT handles the R0:R2 pair, and the remaining ones are left for
future work. Only the x86-64, arm64 and riscv64 JITs opt in so far.

On arm64 and riscv64 the native second return register is already BPF R2
(x1 in bpf2a64[] and a1 in regmap[] respectively), so the upper half needs
no move at all, unlike x86-64's RDX->RSI. The lower half is covered by the
move into BPF R0 that those JITs already emit after every call, from x0
into x8 and from a0 into a5. This has been tested on x86-64 and arm64. The
riscv64 path is expected to work by the same register-mapping reasoning as
arm64 but has not been tested.

bpf_add_kfunc_call() also rejects a kfunc that is marked KF_FASTCALL and
returns more than 8 bytes. The bpf_fastcall contract implemented by
mark_fastcall_pattern_for_call() assumes a call clobbers R0 plus the
registers holding its arguments, so a return in the R0:R2 pair would
clobber an R2 the caller expects the fastcall pattern to preserve. Such
a kfunc is rejected with -EOPNOTSUPP as well.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20260819055254.3295610-1-yonghong.song@linux.dev
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>

0 files changed, 0 insertions, 0 deletions