[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>

1 files changed, 2 insertions, 4 deletions

diff --git a/arch/i386/kernel/signal.c b/arch/i386/kernel/signal.c
index 43002cfb40c4..65d7620eaa09 100644
--- a/arch/i386/kernel/signal.c
+++ b/arch/i386/kernel/signal.c
@@ -128,7 +128,7 @@ restore_sigcontext(struct pt_regs *regs, struct sigcontext __user *sc, int *peax
                          X86_EFLAGS_TF | X86_EFLAGS_SF | X86_EFLAGS_ZF | \
                          X86_EFLAGS_AF | X86_EFLAGS_PF | X86_EFLAGS_CF)
 
-        GET_SEG(gs);
+        COPY_SEG(gs);
         GET_SEG(fs);
         COPY_SEG(es);
         COPY_SEG(ds);
@@ -244,9 +244,7 @@ setup_sigcontext(struct sigcontext __user *sc, struct _fpstate __user *fpstate,
 {
         int tmp, err = 0;
 
-        tmp = 0;
-        savesegment(gs, tmp);
-        err |= __put_user(tmp, (unsigned int __user *)&sc->gs);
+        err |= __put_user(regs->xgs, (unsigned int __user *)&sc->gs);
         savesegment(fs, tmp);
         err |= __put_user(tmp, (unsigned int __user *)&sc->fs);