There is no LoongArch support for get_preempt_count() currently and its
fallback path always returns 0, just add it so that bpf_in_interrupt(),
bpf_in_nmi(), bpf_in_hardirq(), bpf_in_serving_softirq(), bpf_in_task()
work for LoongArch as well.

Since LoongArch is transitioning to CONFIG_THREAD_INFO_IN_TASK, use
bpf_core_field_exists() to provide compatibility for both legacy and
future kernels. A struct flavor "task_struct___local" is introduced
to bypass static compiler checks when thread_info is missing from the
base task_struct.

For the newer kernels that select CONFIG_THREAD_INFO_IN_TASK, it reads
preempt_count from the thread_info embedded within task_struct. For the
older kernels without selecting CONFIG_THREAD_INFO_IN_TASK, it retrieves
the thread_info pointer from the bottom of the kernel stack (task->stack)
and then reads the preempt_count. This ensures the BPF selftests remain
functional across various LoongArch kernel versions.

With this patch, "./test_progs -t exe_ctx" passes on LoongArch regardless
of whether CONFIG_THREAD_INFO_IN_TASK is set.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
---
 .../testing/selftests/bpf/bpf_experimental.h  | 23 +++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/tools/testing/selftests/bpf/bpf_experimental.h b/tools/testing/selftests/bpf/bpf_experimental.h
index d1db355e872b..1d0b29a9acb8 100644
--- a/tools/testing/selftests/bpf/bpf_experimental.h
+++ b/tools/testing/selftests/bpf/bpf_experimental.h
@@ -401,6 +401,12 @@ struct task_struct___preempt_rt {
 extern struct lowcore *bpf_get_lowcore(void) __weak __ksym;
 #endif
 
+#ifdef bpf_target_loongarch
+struct task_struct___local {
+	struct thread_info thread_info;
+} __attribute__((preserve_access_index));
+#endif
+
 static inline int get_preempt_count(void)
 {
 #if defined(bpf_target_x86)
@@ -423,6 +429,18 @@ static inline int get_preempt_count(void)
 	return bpf_get_current_task_btf()->thread_info.preempt_count;
 #elif defined(bpf_target_s390)
 	return bpf_get_lowcore()->preempt_count;
+#elif defined(bpf_target_loongarch)
+	struct task_struct *task = bpf_get_current_task_btf();
+	struct task_struct___local *task_alt = (void *)task;
+
+	if (bpf_core_field_exists(task_alt->thread_info)) {
+		return BPF_CORE_READ(task_alt, thread_info.preempt_count);
+	} else {
+		void *stack = BPF_CORE_READ(task, stack);
+		struct thread_info *ti = (void *)stack;
+
+		return BPF_CORE_READ(ti, preempt_count);
+	}
 #endif
 	return 0;
 }
@@ -433,6 +451,7 @@ static inline int get_preempt_count(void)
  *	* arm64
  *	* powerpc64
  *	* s390x
+ *	* loongarch
  */
 static inline int bpf_in_interrupt(void)
 {
@@ -454,6 +473,7 @@ static inline int bpf_in_interrupt(void)
  *	* arm64
  *	* powerpc64
  *	* s390x
+ *	* loongarch
  */
 static inline int bpf_in_nmi(void)
 {
@@ -466,6 +486,7 @@ static inline int bpf_in_nmi(void)
  *	* arm64
  *	* powerpc64
  *	* s390x
+ *	* loongarch
  */
 static inline int bpf_in_hardirq(void)
 {
@@ -478,6 +499,7 @@ static inline int bpf_in_hardirq(void)
  *	* arm64
  *	* powerpc64
  *	* s390x
+ *	* loongarch
  */
 static inline int bpf_in_serving_softirq(void)
 {
@@ -498,6 +520,7 @@ static inline int bpf_in_serving_softirq(void)
  *	* arm64
  *	* powerpc64
  *	* s390x
+ *	* loongarch
  */
 static inline int bpf_in_task(void)
 {
-- 
2.42.0