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

[Tiezhu Yang]

On 11/4/26 12:29 PM, Huacai Chen wrote:
> On Tue, Nov 3, 2026 at 10:24 AM Tiezhu Yang <yangtiezhu@xxxxxxxxxxx> wrote:

...

> > So, the current patch has no problem IMO.
> OK, then I think we can use:
>
> return __is_defined(system_has_cmpxchg128) && system_has_cmpxchg128();

If CONFIG_AS_HAS_SCQ_EXTENSION is not enabled, system_has_cmpxchg128()
is not defined at all. If we change the code to follow your suggestion,
something like this:

```
diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index b5ed4881d3c0..6f572d24d2c3 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -336,7 +336,7 @@ config AS_HAS_LVZ_EXTENSION
        def_bool $(as-instr,hvcl 0)

 config AS_HAS_SCQ_EXTENSION
-       def_bool $(as-instr,sc.q \$t0$(comma)\$t1$(comma)\$t2)
+       def_bool n

 config CC_HAS_ANNOTATE_TABLEJUMP
        def_bool $(cc-option,-mannotate-tablejump)
diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index d7ea515045ec..d124632b0fa9 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -2495,11 +2495,7 @@ bool bpf_jit_supports_arena(void)
         * atomic cmpxchg; this is provided by not only hardware SCQ 
extension
         * but also assembler support on LoongArch.
         */
-#ifdef system_has_cmpxchg128
-       return system_has_cmpxchg128();
-#else
-       return false;
-#endif
+       return __is_defined(system_has_cmpxchg128) && 
system_has_cmpxchg128();
 }

 bool bpf_jit_supports_fsession(void)
```

We will encounter a fatal build error:

  CC      arch/loongarch/net/bpf_jit.o
arch/loongarch/net/bpf_jit.c: In function ‘bpf_jit_supports_arena’:
arch/loongarch/net/bpf_jit.c:2498:55: error: implicit declaration of 
function ‘system_has_cmpxchg128’ [-Wimplicit-function-declaration]
 2498 |         return __is_defined(system_has_cmpxchg128) && 
system_has_cmpxchg128();
      | 
^~~~~~~~~~~~~~~~~~~~~
make[5]: *** [scripts/Makefile.build:290: arch/loongarch/net/bpf_jit.o] 
Error 1

Even if we change it to use IS_ENABLED(CONFIG_AS_HAS_SCQ_EXTENSION),
something like this:

```
diff --git a/arch/loongarch/Kconfig b/arch/loongarch/Kconfig
index b5ed4881d3c0..6f572d24d2c3 100644
--- a/arch/loongarch/Kconfig
+++ b/arch/loongarch/Kconfig
@@ -336,7 +336,7 @@ config AS_HAS_LVZ_EXTENSION
        def_bool $(as-instr,hvcl 0)

 config AS_HAS_SCQ_EXTENSION
-       def_bool $(as-instr,sc.q \$t0$(comma)\$t1$(comma)\$t2)
+       def_bool n

 config CC_HAS_ANNOTATE_TABLEJUMP
        def_bool $(cc-option,-mannotate-tablejump)
diff --git a/arch/loongarch/net/bpf_jit.c b/arch/loongarch/net/bpf_jit.c
index d7ea515045ec..1b87b1e4262e 100644
--- a/arch/loongarch/net/bpf_jit.c
+++ b/arch/loongarch/net/bpf_jit.c
@@ -2495,11 +2495,7 @@ bool bpf_jit_supports_arena(void)
         * atomic cmpxchg; this is provided by not only hardware SCQ 
extension
         * but also assembler support on LoongArch.
         */
-#ifdef system_has_cmpxchg128
-       return system_has_cmpxchg128();
-#else
-       return false;
-#endif
+       return IS_ENABLED(CONFIG_AS_HAS_SCQ_EXTENSION) && 
system_has_cmpxchg128();
 }

 bool bpf_jit_supports_fsession(void)
```

It still fails with the same error due to the macro visibility:

  CC      arch/loongarch/net/bpf_jit.o
arch/loongarch/net/bpf_jit.c: In function ‘bpf_jit_supports_arena’:
arch/loongarch/net/bpf_jit.c:2498:59: error: implicit declaration of 
function ‘system_has_cmpxchg128’ [-Wimplicit-function-declaration]
 2498 |         return IS_ENABLED(CONFIG_AS_HAS_SCQ_EXTENSION) && 
system_has_cmpxchg128();
      | 
^~~~~~~~~~~~~~~~~~~~~
make[5]: *** [scripts/Makefile.build:290: arch/loongarch/net/bpf_jit.o] 
Error 1

Therefore, using the explicit #ifdef block in the original patch is the
only way to ensure toolchain compatibility and compile-time robustness
here IMO.

Anyway, I will modify the patch subject, update the commit message
and code comment to make it more clear.

Thanks,
Tiezhu