Subject: [PATCH 3/8] powerpc: use switch frame for ret_from_kernel_thread parameters

From: Nicholas Piggin <[email-masked]>
Date: Wed, 1 Feb 2023 02:55:29 +1000

The kernel thread path in copy_thread creates a user interrupt frame on
stack and stores the function and arg parameters there, and
ret_from_kernel_thread loads them. This is a slightly confusing way to
overload that frame. Non-volatile registers are loaded from the switch
frame, so the parameters can be stored there.

Signed-off-by: Nicholas Piggin <[email-masked]>
---
 arch/powerpc/kernel/entry_32.S     | 1 -
 arch/powerpc/kernel/interrupt_64.S | 1 -
 arch/powerpc/kernel/process.c      | 9 +++++++++
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
index 755408c63be8..c3fdb3081d3d 100644
--- a/arch/powerpc/kernel/entry_32.S
+++ b/arch/powerpc/kernel/entry_32.S
@@ -188,7 +188,6 @@ ret_from_fork:
 
 	.globl	ret_from_kernel_thread
 ret_from_kernel_thread:
-	REST_NVGPRS(r1)
 	bl	schedule_tail
 	mtctr	r14
 	mr	r3,r15
diff --git a/arch/powerpc/kernel/interrupt_64.S b/arch/powerpc/kernel/interrupt_64.S
index fccc34489add..d60e7e7564df 100644
--- a/arch/powerpc/kernel/interrupt_64.S
+++ b/arch/powerpc/kernel/interrupt_64.S
@@ -741,7 +741,6 @@ _GLOBAL(ret_from_fork)
 
 _GLOBAL(ret_from_kernel_thread)
 	bl	schedule_tail
-	REST_NVGPRS(r1)
 	mtctr	r14
 	mr	r3,r15
 #ifdef CONFIG_PPC64_ELF_ABI_V2
diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
index dc66ca668b44..6cea224b7e60 100644
--- a/arch/powerpc/kernel/process.c
+++ b/arch/powerpc/kernel/process.c
@@ -1812,6 +1812,15 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
 	((unsigned long *)sp)[0] = sp + STACK_SWITCH_FRAME_SIZE;
 	kregs = (struct pt_regs *)(sp + STACK_SWITCH_FRAME_REGS);
 	kregs->nip = ppc_function_entry(f);
+	if (unlikely(args->fn)) {
+		/*
+		 * Put kthread create details in non-volatile GPRs in the
+		 * switch frame so they are loaded by _switch before it
+		 * returns to ret_from_kernel_thread.
+		 */
+		kregs->gpr[14] = ppc_function_entry((void *)args->fn);
+		kregs->gpr[15] = (unsigned long)args->fn_arg;
+	}
 	p->thread.ksp = sp;
 
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
-- 
2.37.2