[PATCH] i386: Convert PDA into the percpu section

Currently x86 (similar to x84-64) has a special per-cpu structure
called "i386_pda" which can be easily and efficiently referenced via
the %fs register.  An ELF section is more flexible than a structure,
allowing any piece of code to use this area.  Indeed, such a section
already exists: the per-cpu area.

So this patch:
(1) Removes the PDA and uses per-cpu variables for each current member.
(2) Replaces the __KERNEL_PDA segment with __KERNEL_PERCPU.
(3) Creates a per-cpu mirror of __per_cpu_offset called this_cpu_off, which
    can be used to calculate addresses for this CPU's variables.
(4) Simplifies startup, because %fs doesn't need to be loaded with a
    special segment at early boot; it can be deferred until the first
    percpu area is allocated (or never for UP).

The result is less code and one less x86-specific concept.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Andi Kleen <ak@suse.de>

1 files changed, 6 insertions, 25 deletions

diff --git a/arch/i386/kernel/head.S b/arch/i386/kernel/head.S
index bb36c24311b4..12277d8938df 100644
--- a/arch/i386/kernel/head.S
+++ b/arch/i386/kernel/head.S
@@ -317,12 +317,12 @@ is386:	movl $2,%ecx		# set MP
         movl %eax,%cr0
 
         call check_x87
-        call setup_pda
         lgdt early_gdt_descr
         lidt idt_descr
         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
@@ -332,16 +332,17 @@ is386:	movl $2,%ecx		# set MP
         movl %eax,%gs
         lldt %ax
 
-        movl $(__KERNEL_PDA),%eax
-        mov  %eax,%fs
-
         cld                     # gcc2 wants the direction flag cleared at all times
         pushl $0                # fake return address for unwinder
 #ifdef CONFIG_SMP
         movb ready, %cl
         movb $1, ready
         cmpb $0,%cl             # the first CPU calls start_kernel
-        jne initialize_secondary # all other CPUs call initialize_secondary
+        je   1f
+        movl $(__KERNEL_PERCPU), %eax
+        movl %eax,%fs           # set this cpu's percpu
+        jmp initialize_secondary # all other CPUs call initialize_secondary
+1:
 #endif /* CONFIG_SMP */
         jmp start_kernel
 
@@ -365,23 +366,6 @@ check_x87:
         ret
 
 /*
- * Point the GDT at this CPU's PDA.  On boot this will be
- * cpu_gdt_table and boot_pda; for secondary CPUs, these will be
- * that CPU's GDT and PDA.
- */
-ENTRY(setup_pda)
-        /* get the PDA pointer */
-        movl start_pda, %eax
-
-        /* slot the PDA address into the GDT */
-        mov early_gdt_descr+2, %ecx
-        mov %ax, (__KERNEL_PDA+0+2)(%ecx)               /* base & 0x0000ffff */
-        shr $16, %eax
-        mov %al, (__KERNEL_PDA+4+0)(%ecx)               /* base & 0x00ff0000 */
-        mov %ah, (__KERNEL_PDA+4+3)(%ecx)               /* base & 0xff000000 */
-        ret
-
-/*
  *  setup_idt
  *
  *  sets up a idt with 256 entries pointing to
@@ -553,9 +537,6 @@ ENTRY(empty_zero_page)
  * This starts the data section.
  */
 .data
-ENTRY(start_pda)
-        .long boot_pda
-
 ENTRY(stack_start)
         .long init_thread_union+THREAD_SIZE
         .long __BOOT_DS