[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, 12 insertions, 14 deletions

diff --git a/arch/i386/kernel/process.c b/arch/i386/kernel/process.c
index ae924c416b68..905364d42847 100644
--- a/arch/i386/kernel/process.c
+++ b/arch/i386/kernel/process.c
@@ -56,6 +56,7 @@
 
 #include <asm/tlbflush.h>
 #include <asm/cpu.h>
+#include <asm/pda.h>
 
 asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");
 
@@ -346,6 +347,7 @@ int kernel_thread(int (*fn)(void *), void * arg, unsigned long flags)
 
 	regs.xds = __USER_DS;
 	regs.xes = __USER_DS;
+	regs.xgs = __KERNEL_PDA;
 	regs.orig_eax = -1;
 	regs.eip = (unsigned long) kernel_thread_helper;
 	regs.xcs = __KERNEL_CS | get_kernel_rpl();
@@ -431,7 +433,6 @@ int copy_thread(int nr, unsigned long clone_flags, unsigned long esp,
 	p->thread.eip = (unsigned long) ret_from_fork;
 
 	savesegment(fs,p->thread.fs);
-	savesegment(gs,p->thread.gs);
 
 	tsk = current;
 	if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
@@ -659,16 +660,16 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
 	load_esp0(tss, next);
 
 	/*
-	 * Save away %fs and %gs. No need to save %es and %ds, as
-	 * those are always kernel segments while inside the kernel.
-	 * Doing this before setting the new TLS descriptors avoids
-	 * the situation where we temporarily have non-reloadable
-	 * segments in %fs and %gs.  This could be an issue if the
-	 * NMI handler ever used %fs or %gs (it does not today), or
-	 * if the kernel is running inside of a hypervisor layer.
+	 * Save away %fs. No need to save %gs, as it was saved on the
+	 * stack on entry.  No need to save %es and %ds, as those are
+	 * always kernel segments while inside the kernel.  Doing this
+	 * before setting the new TLS descriptors avoids the situation
+	 * where we temporarily have non-reloadable segments in %fs
+	 * and %gs.  This could be an issue if the NMI handler ever
+	 * used %fs or %gs (it does not today), or if the kernel is
+	 * running inside of a hypervisor layer.
 	 */
 	savesegment(fs, prev->fs);
-	savesegment(gs, prev->gs);
 
 	/*
 	 * Load the per-thread Thread-Local Storage descriptor.
@@ -676,16 +677,13 @@ struct task_struct fastcall * __switch_to(struct task_struct *prev_p, struct tas
 	load_TLS(next, cpu);
 
 	/*
-	 * Restore %fs and %gs if needed.
+	 * Restore %fs if needed.
 	 *
-	 * Glibc normally makes %fs be zero, and %gs is one of
-	 * the TLS segments.
+	 * Glibc normally makes %fs be zero.
 	 */
 	if (unlikely(prev->fs | next->fs))
 		loadsegment(fs, next->fs);
 
-	if (prev->gs | next->gs)
-		loadsegment(gs, next->gs);
 
 	/*
 	 * Restore IOPL if needed.