Merge tag 'core-entry-2025-05-25' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull core entry code updates from Thomas Gleixner:
 "Updates for the generic and architecture entry code:

   - Move LoongArch and RISC-V ret_from_fork() implementations to C code
     so that syscall_exit_user_mode() can be inlined

   - Split the RISC-V ret_from_fork() implementation into return to user
     and return to kernel, which gives a measurable performance
     improvement

   - Inline syscall_exit_user_mode() which benefits all architectures by
     avoiding a function call and letting the compiler do better
     optimizations"

* tag 'core-entry-2025-05-25' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  LoongArch: entry: Fix include order
  entry: Inline syscall_exit_to_user_mode()
  LoongArch: entry: Migrate ret_from_fork() to C
  riscv: entry: Split ret_from_fork() into user and kernel
  riscv: entry: Convert ret_from_fork() to C

2 files changed, 30 insertions, 11 deletions

diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
index 33a5a9f2a0d4..0fb338000c6d 100644
--- a/arch/riscv/kernel/entry.S
+++ b/arch/riscv/kernel/entry.S
@@ -319,17 +319,21 @@ SYM_CODE_END(handle_kernel_stack_overflow)
 ASM_NOKPROBE(handle_kernel_stack_overflow)
 #endif
 
-SYM_CODE_START(ret_from_fork)
+SYM_CODE_START(ret_from_fork_kernel_asm)
+	call schedule_tail
+	move a0, s1 /* fn_arg */
+	move a1, s0 /* fn */
+	move a2, sp /* pt_regs */
+	call ret_from_fork_kernel
+	j ret_from_exception
+SYM_CODE_END(ret_from_fork_kernel_asm)
+
+SYM_CODE_START(ret_from_fork_user_asm)
 	call schedule_tail
-	beqz s0, 1f	/* not from kernel thread */
-	/* Call fn(arg) */
-	move a0, s1
-	jalr s0
-1:
 	move a0, sp /* pt_regs */
-	call syscall_exit_to_user_mode
+	call ret_from_fork_user
 	j ret_from_exception
-SYM_CODE_END(ret_from_fork)
+SYM_CODE_END(ret_from_fork_user_asm)
 
 #ifdef CONFIG_IRQ_STACKS
 /*
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 15d8f75902f8..bbf7ec6a75c0 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -17,7 +17,9 @@
 #include <linux/ptrace.h>
 #include <linux/uaccess.h>
 #include <linux/personality.h>
+#include <linux/entry-common.h>
 
+#include <asm/asm-prototypes.h>
 #include <asm/unistd.h>
 #include <asm/processor.h>
 #include <asm/csr.h>
@@ -36,7 +38,8 @@ unsigned long __stack_chk_guard __read_mostly;
 EXPORT_SYMBOL(__stack_chk_guard);
 #endif
 
-extern asmlinkage void ret_from_fork(void);
+extern asmlinkage void ret_from_fork_kernel_asm(void);
+extern asmlinkage void ret_from_fork_user_asm(void);
 
 void noinstr arch_cpu_idle(void)
 {
@@ -206,6 +209,18 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
 	return 0;
 }
 
+asmlinkage void ret_from_fork_kernel(void *fn_arg, int (*fn)(void *), struct pt_regs *regs)
+{
+	fn(fn_arg);
+
+	syscall_exit_to_user_mode(regs);
+}
+
+asmlinkage void ret_from_fork_user(struct pt_regs *regs)
+{
+	syscall_exit_to_user_mode(regs);
+}
+
 int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 {
 	unsigned long clone_flags = args->flags;
@@ -228,6 +243,7 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 
 		p->thread.s[0] = (unsigned long)args->fn;
 		p->thread.s[1] = (unsigned long)args->fn_arg;
+		p->thread.ra = (unsigned long)ret_from_fork_kernel_asm;
 	} else {
 		*childregs = *(current_pt_regs());
 		/* Turn off status.VS */
@@ -237,12 +253,11 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 		if (clone_flags & CLONE_SETTLS)
 			childregs->tp = tls;
 		childregs->a0 = 0; /* Return value of fork() */
-		p->thread.s[0] = 0;
+		p->thread.ra = (unsigned long)ret_from_fork_user_asm;
 	}
 	p->thread.riscv_v_flags = 0;
 	if (has_vector() || has_xtheadvector())
 		riscv_v_thread_alloc(p);
-	p->thread.ra = (unsigned long)ret_from_fork;
 	p->thread.sp = (unsigned long)childregs; /* kernel sp */
 	return 0;
 }