[RFC][PATCH 05/24] x86/entry: Implement ret_from_fork body with C code

[Alexandre Chartre <alexandre.chartre@oracle.com>]

ret_from_fork is a mix of assembly code and calls to C functions.
Re-implement ret_from_fork so that it calls a single C function.

Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
---
 arch/x86/entry/common.c   | 18 ++++++++++++++++++
 arch/x86/entry/entry_64.S | 28 +++++-----------------------
 2 files changed, 23 insertions(+), 23 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index d222212908ad..7ee15a12c115 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -35,6 +35,24 @@
 #include <asm/syscall.h>
 #include <asm/irq_stack.h>
 
+__visible noinstr void return_from_fork(struct pt_regs *regs,
+					struct task_struct *prev,
+					void (*kfunc)(void *), void *kargs)
+{
+	schedule_tail(prev);
+	if (kfunc) {
+		/* kernel thread */
+		kfunc(kargs);
+		/*
+		 * A kernel thread is allowed to return here after
+		 * successfully calling kernel_execve(). Exit to
+		 * userspace to complete the execve() syscall.
+		 */
+		regs->ax = 0;
+	}
+	syscall_exit_to_user_mode(regs);
+}
+
 static __always_inline void run_syscall(sys_call_ptr_t sysfunc,
 					struct pt_regs *regs)
 {
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 274384644b5e..73e9cd47dc83 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -276,31 +276,13 @@ SYM_FUNC_END(__switch_to_asm)
  */
 .pushsection .text, "ax"
 SYM_CODE_START(ret_from_fork)
-	UNWIND_HINT_EMPTY
-	movq	%rax, %rdi
-	call	schedule_tail			/* rdi: 'prev' task parameter */
-
-	testq	%rbx, %rbx			/* from kernel_thread? */
-	jnz	1f				/* kernel threads are uncommon */
-
-2:
 	UNWIND_HINT_REGS
-	movq	%rsp, %rdi
-	call	syscall_exit_to_user_mode	/* returns with IRQs disabled */
+	movq	%rsp, %rdi			/* pt_regs */
+	movq	%rax, %rsi			/* 'prev' task parameter */
+	movq	%rbx, %rdx			/* kernel thread func */
+	movq	%r12, %rcx			/* kernel thread arg */
+	call	return_from_fork		/* returns with IRQs disabled */
 	jmp	swapgs_restore_regs_and_return_to_usermode
-
-1:
-	/* kernel thread */
-	UNWIND_HINT_EMPTY
-	movq	%r12, %rdi
-	CALL_NOSPEC rbx
-	/*
-	 * A kernel thread is allowed to return here after successfully
-	 * calling kernel_execve().  Exit to userspace to complete the execve()
-	 * syscall.
-	 */
-	movq	$0, RAX(%rsp)
-	jmp	2b
 SYM_CODE_END(ret_from_fork)
 .popsection
 
-- 
2.18.4