x86: use stack_start in x86_64

call x86_64's init_rsp stack_start, just as i386 does.
Put a zeroed stack segment for consistency. With this,
we can eliminate one ugly ifdef in smpboot.c.
Signed-off-by: NGlauber Costa <gcosta@redhat.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/acpi/sleep.c

@@ -86,7 +86,7 @@ int acpi_save_state_mem(void)
 	saved_magic = 0x12345678;
 #else /* CONFIG_64BIT */
 	header->trampoline_segment = setup_trampoline() >> 4;
-	init_rsp = (unsigned long)temp_stack + 4096;
+	stack_start.sp = temp_stack + 4096;
 	initial_code = (unsigned long)wakeup_long64;
 	saved_magic = 0x123456789abcdef0;
 #endif /* CONFIG_64BIT */

arch/x86/kernel/head_64.S

@@ -191,7 +191,7 @@ ENTRY(secondary_startup_64)
 	movq	%rax, %cr0
 
 	/* Setup a boot time stack */
-	movq init_rsp(%rip),%rsp
+	movq stack_start(%rip),%rsp
 
 	/* zero EFLAGS after setting rsp */
 	pushq $0
@@ -252,8 +252,9 @@ ENTRY(secondary_startup_64)
 	.quad	x86_64_start_kernel
 	__FINITDATA
 
-	ENTRY(init_rsp)
+	ENTRY(stack_start)
 	.quad  init_thread_union+THREAD_SIZE-8
+	.word  0
 
 bad_address:
 	jmp bad_address

arch/x86/kernel/smpboot.c

@@ -714,11 +714,7 @@ wakeup_secondary_cpu(int phys_apicid, unsigned long start_eip)
 	 * target processor state.
 	 */
 	startup_ipi_hook(phys_apicid, (unsigned long) start_secondary,
-#ifdef CONFIG_X86_64
-			 (unsigned long)init_rsp);
-#else
 			 (unsigned long)stack_start.sp);
-#endif
 
 	/*
 	 * Run STARTUP IPI loop.
@@ -905,15 +901,14 @@ static int __cpuinit do_boot_cpu(int apicid, int cpu)
 	early_gdt_descr.address = (unsigned long)get_cpu_gdt_table(cpu);
 	c_idle.idle->thread.ip = (unsigned long) start_secondary;
 	/* Stack for startup_32 can be just as for start_secondary onwards */
-	stack_start.sp = (void *) c_idle.idle->thread.sp;
 	irq_ctx_init(cpu);
 #else
 	cpu_pda(cpu)->pcurrent = c_idle.idle;
-	init_rsp = c_idle.idle->thread.sp;
 	load_sp0(&per_cpu(init_tss, cpu), &c_idle.idle->thread);
 	initial_code = (unsigned long)start_secondary;
 	clear_tsk_thread_flag(c_idle.idle, TIF_FORK);
 #endif
+	stack_start.sp = (void *) c_idle.idle->thread.sp;
 
 	/* start_ip had better be page-aligned! */
 	start_ip = setup_trampoline();