[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, 14 insertions, 16 deletions

diff --git a/arch/i386/kernel/smpboot.c b/arch/i386/kernel/smpboot.c
index 61e2842add36..f79b6233db78 100644
--- a/arch/i386/kernel/smpboot.c
+++ b/arch/i386/kernel/smpboot.c
@@ -53,7 +53,6 @@
 #include <asm/desc.h>
 #include <asm/arch_hooks.h>
 #include <asm/nmi.h>
-#include <asm/pda.h>
 
 #include <mach_apic.h>
 #include <mach_wakecpu.h>
@@ -99,6 +98,9 @@ EXPORT_SYMBOL(x86_cpu_to_apicid);
 
 u8 apicid_2_node[MAX_APICID];
 
+DEFINE_PER_CPU(unsigned long, this_cpu_off);
+EXPORT_PER_CPU_SYMBOL(this_cpu_off);
+
 /*
  * Trampoline 80x86 program as an array.
  */
@@ -456,7 +458,6 @@ extern struct {
         void * esp;
         unsigned short ss;
 } stack_start;
-extern struct i386_pda *start_pda;
 
 #ifdef CONFIG_NUMA
 
@@ -784,20 +785,17 @@ static inline struct task_struct * alloc_idle_task(int cpu)
 /* Initialize the CPU's GDT.  This is either the boot CPU doing itself
    (still using the master per-cpu area), or a CPU doing it for a
    secondary which will soon come up. */
-static __cpuinit void init_gdt(int cpu, struct task_struct *idle)
+static __cpuinit void init_gdt(int cpu)
 {
         struct desc_struct *gdt = get_cpu_gdt_table(cpu);
-        struct i386_pda *pda = &per_cpu(_cpu_pda, cpu);
 
-        pack_descriptor((u32 *)&gdt[GDT_ENTRY_PDA].a,
-                        (u32 *)&gdt[GDT_ENTRY_PDA].b,
-                        (unsigned long)pda, sizeof(*pda) - 1,
-                        0x80 | DESCTYPE_S | 0x2, 0); /* present read-write data segment */
+        pack_descriptor((u32 *)&gdt[GDT_ENTRY_PERCPU].a,
+                        (u32 *)&gdt[GDT_ENTRY_PERCPU].b,
+                        __per_cpu_offset[cpu], 0xFFFFF,
+                        0x80 | DESCTYPE_S | 0x2, 0x8);
 
-        memset(pda, 0, sizeof(*pda));
-        pda->_pda = pda;
-        pda->cpu_number = cpu;
-        pda->pcurrent = idle;
+        per_cpu(this_cpu_off, cpu) = __per_cpu_offset[cpu];
+        per_cpu(cpu_number, cpu) = cpu;
 }
 
 /* Defined in head.S */
@@ -824,9 +822,9 @@ static int __cpuinit do_boot_cpu(int apicid, int cpu)
         if (IS_ERR(idle))
                 panic("failed fork for CPU %d", cpu);
 
-        init_gdt(cpu, idle);
+        init_gdt(cpu);
+        per_cpu(current_task, cpu) = idle;
         early_gdt_descr.address = (unsigned long)get_cpu_gdt_table(cpu);
-        start_pda = cpu_pda(cpu);
 
         idle->thread.eip = (unsigned long) start_secondary;
         /* start_eip had better be page-aligned! */
@@ -1188,14 +1186,14 @@ static inline void switch_to_new_gdt(void)
         gdt_descr.address = (long)get_cpu_gdt_table(smp_processor_id());
         gdt_descr.size = GDT_SIZE - 1;
         load_gdt(&gdt_descr);
-        asm volatile ("mov %0, %%fs" : : "r" (__KERNEL_PDA) : "memory");
+        asm("mov %0, %%fs" : : "r" (__KERNEL_PERCPU) : "memory");
 }
 
 void __init native_smp_prepare_boot_cpu(void)
 {
         unsigned int cpu = smp_processor_id();
 
-        init_gdt(cpu, current);
+        init_gdt(cpu);
         switch_to_new_gdt();
 
         cpu_set(cpu, cpu_online_map);