[PATCH v1 1/2] LoongArch: BPF: Make arena support depend on SCQ extension

[Tiezhu Yang]

When testing bpf selftest "jit_insns/cast_to_user", there are different
behaviors on different Loongson machines.

(1) Loongson-3A6000
  $ sudo ./test_progs -a jit_insns/cast_to_user
  #193/2   jit_insns/cast_to_user:OK
  #193     jit_insns:OK
  Summary: 1/1 PASSED, 0 SKIPPED, 0/0 FAILED

(2) Loongson-3A5000
  $ sudo ./test_progs -a jit_insns/cast_to_user
  ...
  libbpf: map 'arena': failed to create: -ENOMEM
  run_subtest:FAIL:unexpected_load_failure unexpected error: -12 (errno 12)
  ...
  #193/2   jit_insns/cast_to_user:FAIL
  #193     jit_insns:FAIL
  Summary: 0/0 PASSED, 0 SKIPPED, 1/1 FAILED

This is because the arena range tree uses kmalloc_nolock(), which requires
128-bit atomic cmpxchg support, provided by SCQ extension on LoongArch. On
CPUs without the SCQ extension, arena map creation fails with a misleading
-ENOMEM error.

Make bpf_jit_supports_arena() return system_has_cmpxchg128() where it is
defined, so arena map creation fails with -EOPNOTSUPP on CPUs without SCQ
extension.

Furthermore, given that the macro system_has_cmpxchg128() is only defined
when CONFIG_AS_HAS_SCQ_EXTENSION is enabled, guard it with an #ifdef to
preserve compile-time robustness and reject arena otherwise.

With this patch, here is the test result on Loongson-3A5000 without SCQ
extension:

  $ sudo ./test_progs -a jit_insns/cast_to_user
  ...
  libbpf: map 'arena': failed to create: -EOPNOTSUPP
  run_subtest:FAIL:unexpected_load_failure unexpected error: -95 (errno 95)
  ...
  #193/2   jit_insns/cast_to_user:FAIL
  #193     jit_insns:FAIL
  Summary: 0/0 PASSED, 0 SKIPPED, 1/1 FAILED

It is better to mark the test status as SKIPPED rather than logging a
false-alarm failure if the feature is not supported, but this belongs
to a separate bpf selftest patch.

This is similar to commit 536b523b4073 ("bpf, riscv: Make arena support
depend on ZACAS").

Fixes: 4fdb5dd8aeba ("LoongArch: BPF: Implement bpf_addr_space_cast instruction")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Tiezhu Yang <yangtiezhu@xxxxxxxxxxx>
---
 arch/loongarch/net/bpf_jit.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index 3ab039d0afb6..01c12da168ee 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -2489,7 +2489,15 @@ bool bpf_jit_bypass_spec_v4(void)
 
 bool bpf_jit_supports_arena(void)
 {
-	return true;
+	/*
+	 * The arena range tree uses kmalloc_nolock(), which requires 128-bit
+	 * atomic cmpxchg support, provided by SCQ extension on LoongArch.
+	 */
+#ifdef system_has_cmpxchg128
+	return system_has_cmpxchg128();
+#else
+	return false;
+#endif
 }
 
 bool bpf_jit_supports_fsession(void)
-- 
2.42.0