[PATCH] i386: Use %gs as the PDA base-segment in the kernel

This patch is the meat of the PDA change.  This patch makes several related
changes:

1: Most significantly, %gs is now used in the kernel.  This means that on
   entry, the old value of %gs is saved away, and it is reloaded with
   __KERNEL_PDA.

2: entry.S constructs the stack in the shape of struct pt_regs, and this
   is passed around the kernel so that the process's saved register
   state can be accessed.

   Unfortunately struct pt_regs doesn't currently have space for %gs
   (or %fs). This patch extends pt_regs to add space for gs (no space
   is allocated for %fs, since it won't be used, and it would just
   complicate the code in entry.S to work around the space).

3: Because %gs is now saved on the stack like %ds, %es and the integer
   registers, there are a number of places where it no longer needs to
   be handled specially; namely context switch, and saving/restoring the
   register state in a signal context.

4: And since kernel threads run in kernel space and call normal kernel
   code, they need to be created with their %gs == __KERNEL_PDA.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Cc: Chuck Ebbert <76306.1226@compuserve.com>
Cc: Zachary Amsden <zach@vmware.com>
Cc: Jan Beulich <jbeulich@novell.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>

3 files changed, 7 insertions, 3 deletions

diff --git a/include/asm-i386/mmu_context.h b/include/asm-i386/mmu_context.h
index 1b1495372c4d..68ff102d6f5e 100644
--- a/include/asm-i386/mmu_context.h
+++ b/include/asm-i386/mmu_context.h
@@ -62,8 +62,8 @@ static inline void switch_mm(struct mm_struct *prev,
 #endif
 }
 
-#define deactivate_mm(tsk, mm) \
-        asm("movl %0,%%fs ; movl %0,%%gs": :"r" (0))
+#define deactivate_mm(tsk, mm)                  \
+        asm("movl %0,%%fs": :"r" (0));
 
 #define activate_mm(prev, next) \
         switch_mm((prev),(next),NULL)
diff --git a/include/asm-i386/processor.h b/include/asm-i386/processor.h
index a9f2041c7c87..f73cf836e649 100644
--- a/include/asm-i386/processor.h
+++ b/include/asm-i386/processor.h
@@ -473,6 +473,7 @@ struct thread_struct {
         .vm86_info = NULL,                                              \
         .sysenter_cs = __KERNEL_CS,                                     \
         .io_bitmap_ptr = NULL,                                          \
+        .gs = __KERNEL_PDA,                                             \
 }
 
 /*
@@ -500,7 +501,8 @@ static inline void load_esp0(struct tss_struct *tss, struct thread_struct *threa
 }
 
 #define start_thread(regs, new_eip, new_esp) do {               \
-        __asm__("movl %0,%%fs ; movl %0,%%gs": :"r" (0));       \
+        __asm__("movl %0,%%fs": :"r" (0));                      \
+        regs->xgs = 0;                                          \
         set_fs(USER_DS);                                        \
         regs->xds = __USER_DS;                                  \
         regs->xes = __USER_DS;                                  \
diff --git a/include/asm-i386/ptrace.h b/include/asm-i386/ptrace.h
index d505f501077a..bdbc894339b4 100644
--- a/include/asm-i386/ptrace.h
+++ b/include/asm-i386/ptrace.h
@@ -16,6 +16,8 @@ struct pt_regs {
         long eax;
         int  xds;
         int  xes;
+        /* int  xfs; */
+        int  xgs;
         long orig_eax;
         long eip;
         int  xcs;