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>

1 files changed, 10 insertions, 5 deletions

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 7ee0363871e8..2f0ab0089883 100644
--- a/arch/x86/kernel/head_64.S
+++ b/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)