[RFC][PATCH v2 05/21] x86/entry: Implement ret_from_fork body with C code

Alexandre Chartre
2020-11-16T14:47:17Z

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