x86: load pointer to pda into %gs while brining up a CPU

[ Based on original patch from Christoph Lameter and Mike Travis. ]

CPU startup code in head_64.S loaded address of a zero page into %gs
for temporary use till pda is loaded but address to the actual pda is
available at the point.  Load the real address directly instead.

This will help unifying percpu and pda handling later on.

This patch is mostly taken from Mike Travis' "x86_64: Fold pda into
per cpu area" patch.
Signed-off-by: Tejun Heo <tj@kernel.org>

arch/x86/include/asm/trampoline.h

@@ -13,6 +13,7 @@ extern unsigned char *trampoline_base;
 
 extern unsigned long init_rsp;
 extern unsigned long initial_code;
+extern unsigned long initial_gs;
 
 #define TRAMPOLINE_SIZE roundup(trampoline_end - trampoline_data, PAGE_SIZE)
 #define TRAMPOLINE_BASE 0x6000

arch/x86/kernel/acpi/sleep.c

@@ -101,6 +101,7 @@ int acpi_save_state_mem(void)
 	stack_start.sp = temp_stack + sizeof(temp_stack);
 	early_gdt_descr.address =
 			(unsigned long)get_cpu_gdt_table(smp_processor_id());
+	initial_gs = (unsigned long)cpu_pda(smp_processor_id());
 #endif
 	initial_code = (unsigned long)wakeup_long64;
 	saved_magic = 0x123456789abcdef0;

arch/x86/kernel/head64.c

@@ -26,8 +26,8 @@
 #include <asm/bios_ebda.h>
 #include <asm/trampoline.h>
 
-/* boot cpu pda */
-static struct x8664_pda _boot_cpu_pda;
+/* boot cpu pda, referenced by head_64.S to initialize %gs for boot CPU */
+struct x8664_pda _boot_cpu_pda;
 
 #ifdef CONFIG_SMP
 /*

arch/x86/kernel/head_64.S

@@ -243,12 +243,15 @@ ENTRY(secondary_startup_64)
 	movl %eax,%fs
 	movl %eax,%gs
 
-	/* 
-	 * Setup up a dummy PDA. this is just for some early bootup code
-	 * that does in_interrupt() 
+	/* Set up %gs.
+	 *
+	 * %gs should point to the pda.  For initial boot, make %gs point
+	 * to the _boot_cpu_pda in data section.  For a secondary CPU,
+	 * initial_gs should be set to its pda address before the CPU runs
+	 * this code.
 	 */
 	movl	$MSR_GS_BASE,%ecx
-	movq	$empty_zero_page,%rax
+	movq	initial_gs(%rip),%rax
 	movq    %rax,%rdx
 	shrq	$32,%rdx
 	wrmsr	
@@ -274,6 +277,8 @@ ENTRY(secondary_startup_64)
 	.align	8
 	ENTRY(initial_code)
 	.quad	x86_64_start_kernel
+	ENTRY(initial_gs)
+	.quad	_boot_cpu_pda
 	__FINITDATA
 
 	ENTRY(stack_start)

arch/x86/kernel/smpboot.c

@@ -854,6 +854,7 @@ static int __cpuinit do_boot_cpu(int apicid, int cpu)
 #else
 	cpu_pda(cpu)->pcurrent = c_idle.idle;
 	clear_tsk_thread_flag(c_idle.idle, TIF_FORK);
+	initial_gs = (unsigned long)cpu_pda(cpu);
 #endif
 	early_gdt_descr.address = (unsigned long)get_cpu_gdt_table(cpu);
 	initial_code = (unsigned long)start_secondary;