x86: move stack_canary into irq_stack

Impact: x86_64 percpu area layout change, irq_stack now at the beginning

Now that the PDA is empty except for the stack canary, it can be removed.
The irqstack is moved to the start of the per-cpu section.  If the stack
protector is enabled, the canary overlaps the bottom 48 bytes of the irqstack.

tj: * updated subject
    * dropped asm relocation of irq_stack_ptr
    * updated comments a bit
    * rebased on top of stack canary changes

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

1 files changed, 5 insertions, 8 deletions

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 98ea26a2fca..a0a2b5ca9b7 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -242,13 +242,10 @@ ENTRY(secondary_startup_64)
 
         /* Set up %gs.
          *
-         * On SMP, %gs should point to the per-cpu area.  For initial
-         * boot, make %gs point to the init data section.  For a
-         * secondary CPU,initial_gs should be set to its pda address
-         * before the CPU runs this code.
-         *
-         * On UP, initial_gs points to PER_CPU_VAR(__pda) and doesn't
-         * change.
+         * The base of %gs always points to the bottom of the irqstack
+         * union.  If the stack protector canary is enabled, it is
+         * located at %gs:40.  Note that, on SMP, the boot cpu uses
+         * init data section till per cpu areas are set up.
          */
         movl    $MSR_GS_BASE,%ecx
         movq    initial_gs(%rip),%rax
@@ -281,7 +278,7 @@ ENTRY(secondary_startup_64)
 #ifdef CONFIG_SMP
         .quad   __per_cpu_load
 #else
-        .quad   PER_CPU_VAR(__pda)
+        .quad   PER_CPU_VAR(irq_stack_union)
 #endif
         __FINITDATA