x86: set %fs to __KERNEL_PERCPU unconditionally for x86_32

Impact: cleanup

%fs is currently set to __KERNEL_DS at boot, and conditionally
switched to __KERNEL_PERCPU for secondary cpus.  Instead, initialize
GDT_ENTRY_PERCPU to the same attributes as GDT_ENTRY_KERNEL_DS and
set %fs to __KERNEL_PERCPU unconditionally.

Signed-off-by: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

arch/x86/kernel/cpu/common.c

@@ -111,7 +111,7 @@ DEFINE_PER_CPU_PAGE_ALIGNED(struct gdt_page, gdt_page) = { .gdt = {
 	[GDT_ENTRY_APMBIOS_BASE+2] = { { { 0x0000ffff, 0x00409200 } } },
 
 	[GDT_ENTRY_ESPFIX_SS] = { { { 0x00000000, 0x00c09200 } } },
-	[GDT_ENTRY_PERCPU] = { { { 0x00000000, 0x00000000 } } },
+	[GDT_ENTRY_PERCPU] = { { { 0x0000ffff, 0x00cf9200 } } },
 #endif
 } };
 EXPORT_PER_CPU_SYMBOL_GPL(gdt_page);

arch/x86/kernel/head_32.S

@@ -429,12 +429,14 @@ is386:	movl $2,%ecx		# set MP
 	ljmp $(__KERNEL_CS),$1f
 1:	movl $(__KERNEL_DS),%eax	# reload all the segment registers
 	movl %eax,%ss			# after changing gdt.
-	movl %eax,%fs			# gets reset once there's real percpu
 
 	movl $(__USER_DS),%eax		# DS/ES contains default USER segment
 	movl %eax,%ds
 	movl %eax,%es
 
+	movl $(__KERNEL_PERCPU), %eax
+	movl %eax,%fs			# set this cpu's percpu
+
 	xorl %eax,%eax			# Clear GS and LDT
 	movl %eax,%gs
 	lldt %ax
@@ -446,8 +448,6 @@ is386:	movl $2,%ecx		# set MP
 	movb $1, ready
 	cmpb $0,%cl		# the first CPU calls start_kernel
 	je   1f
-	movl $(__KERNEL_PERCPU), %eax
-	movl %eax,%fs		# set this cpu's percpu
 	movl (stack_start), %esp
 1:
 #endif /* CONFIG_SMP */