x86/smpboot: Rename start_cpu0() to soft_restart_cpu()

This is used in the SEV play_dead() implementation to re-online CPUs. But
that has nothing to do with CPU0.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Tested-by: Helge Deller <deller@gmx.de> # parisc
Tested-by: Guilherme G. Piccoli <gpiccoli@igalia.com> # Steam Deck
Link: https://lore.kernel.org/r/20230512205255.662319599@linutronix.de

arch/x86/include/asm/cpu.h

@@ -30,7 +30,7 @@ struct x86_cpu {
 #ifdef CONFIG_HOTPLUG_CPU
 extern int arch_register_cpu(int num);
 extern void arch_unregister_cpu(int);
-extern void start_cpu0(void);
+extern void soft_restart_cpu(void);
 #ifdef CONFIG_DEBUG_HOTPLUG_CPU0
 extern int _debug_hotplug_cpu(int cpu, int action);
 #endif

arch/x86/kernel/callthunks.c

@@ -134,7 +134,7 @@ static bool skip_addr(void *dest)
 	if (dest == ret_from_fork)
 		return true;
 #ifdef CONFIG_HOTPLUG_CPU
-	if (dest == start_cpu0)
+	if (dest == soft_restart_cpu)
 		return true;
 #endif
 #ifdef CONFIG_FUNCTION_TRACER

arch/x86/kernel/head_32.S

@@ -140,16 +140,16 @@ SYM_CODE_END(startup_32)
 
 #ifdef CONFIG_HOTPLUG_CPU
 /*
- * Boot CPU0 entry point. It's called from play_dead(). Everything has been set
- * up already except stack. We just set up stack here. Then call
- * start_secondary().
+ * Entry point for soft restart of a CPU. Invoked from xxx_play_dead() for
+ * restarting the boot CPU or for restarting SEV guest CPUs after CPU hot
+ * unplug. Everything is set up already except the stack.
  */
-SYM_FUNC_START(start_cpu0)
+SYM_FUNC_START(soft_restart_cpu)
 	movl initial_stack, %ecx
 	movl %ecx, %esp
 	call *(initial_code)
 1:	jmp 1b
-SYM_FUNC_END(start_cpu0)
+SYM_FUNC_END(soft_restart_cpu)
 #endif
 
 /*

arch/x86/kernel/head_64.S

@@ -377,11 +377,11 @@ SYM_CODE_END(secondary_startup_64)
 
 #ifdef CONFIG_HOTPLUG_CPU
 /*
- * Boot CPU0 entry point. It's called from play_dead(). Everything has been set
- * up already except stack. We just set up stack here. Then call
- * start_secondary() via .Ljump_to_C_code.
+ * Entry point for soft restart of a CPU. Invoked from xxx_play_dead() for
+ * restarting the boot CPU or for restarting SEV guest CPUs after CPU hot
+ * unplug. Everything is set up already except the stack.
  */
-SYM_CODE_START(start_cpu0)
+SYM_CODE_START(soft_restart_cpu)
 	ANNOTATE_NOENDBR
 	UNWIND_HINT_END_OF_STACK
 
@@ -390,7 +390,7 @@ SYM_CODE_START(start_cpu0)
 	movq	TASK_threadsp(%rcx), %rsp
 
 	jmp	.Ljump_to_C_code
-SYM_CODE_END(start_cpu0)
+SYM_CODE_END(soft_restart_cpu)
 #endif
 
 #ifdef CONFIG_AMD_MEM_ENCRYPT

arch/x86/kernel/sev.c

@@ -1328,7 +1328,7 @@ static void sev_es_play_dead(void)
 	 * If we get here, the VCPU was woken up again. Jump to CPU
 	 * startup code to get it back online.
 	 */
-	start_cpu0();
+	soft_restart_cpu();
 }
 #else  /* CONFIG_HOTPLUG_CPU */
 #define sev_es_play_dead	native_play_dead