powerpc: Prepare to build with generic entry/exit framework

This patch introduces preparatory changes needed to support building
PowerPC with the generic entry/exit (irqentry) framework.

The following infrastructure updates are added:
 - Add a syscall_work field to struct thread_info to hold SYSCALL_WORK_* flags.
 - Provide a stub implementation of arch_syscall_is_vdso_sigreturn(),
   returning false for now.
 - Introduce on_thread_stack() helper to detect if the current stack pointer
   lies within the task’s kernel stack.

These additions enable later integration with the generic entry/exit
infrastructure while keeping existing PowerPC behavior unchanged.

No functional change is intended in this patch.

Signed-off-by: Mukesh Kumar Chaurasiya <mchauras@linux.ibm.com>
Tested-by: Samir M <samir@linux.ibm.com>
Tested-by: David Gow <davidgow@google.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Reviewed-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260427122742.210074-3-mkchauras@gmail.com

arch/powerpc/include/asm/entry-common.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _ASM_PPC_ENTRY_COMMON_H
+#define _ASM_PPC_ENTRY_COMMON_H
+
+#include <asm/stacktrace.h>
+
+#endif /* _ASM_PPC_ENTRY_COMMON_H */

arch/powerpc/include/asm/stacktrace.h

@@ -10,4 +10,10 @@
 
 void show_user_instructions(struct pt_regs *regs);
 
+static __always_inline bool on_thread_stack(void)
+{
+	return !(((unsigned long)(current->stack) ^ current_stack_pointer)
+			& ~(THREAD_SIZE - 1));
+}
+
 #endif /* _ASM_POWERPC_STACKTRACE_H */

arch/powerpc/include/asm/syscall.h

@@ -139,4 +139,9 @@ static inline int syscall_get_arch(struct task_struct *task)
 	else
 		return AUDIT_ARCH_PPC64;
 }
+
+static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs)
+{
+	return false;
+}
 #endif	/* _ASM_SYSCALL_H */

arch/powerpc/include/asm/thread_info.h

@@ -57,6 +57,7 @@ struct thread_info {
 #ifdef CONFIG_SMP
 	unsigned int	cpu;
 #endif
+	unsigned long	syscall_work;		/* SYSCALL_WORK_ flags */
 	unsigned long	local_flags;		/* private flags for thread */
 #ifdef CONFIG_LIVEPATCH_64
 	unsigned long *livepatch_sp;