[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>

6 files changed, 109 insertions, 46 deletions

diff --git a/arch/i386/kernel/asm-offsets.c b/arch/i386/kernel/asm-offsets.c
index 70b19807acf9..9620872d3534 100644
--- a/arch/i386/kernel/asm-offsets.c
+++ b/arch/i386/kernel/asm-offsets.c
@@ -72,6 +72,7 @@ void foo(void)
         OFFSET(PT_EAX, pt_regs, eax);
         OFFSET(PT_DS,  pt_regs, xds);
         OFFSET(PT_ES,  pt_regs, xes);
+        OFFSET(PT_GS,  pt_regs, xgs);
         OFFSET(PT_ORIG_EAX, pt_regs, orig_eax);
         OFFSET(PT_EIP, pt_regs, eip);
         OFFSET(PT_CS,  pt_regs, xcs);
diff --git a/arch/i386/kernel/cpu/common.c b/arch/i386/kernel/cpu/common.c
index 2534e25ed745..4e63d8ce602b 100644
--- a/arch/i386/kernel/cpu/common.c
+++ b/arch/i386/kernel/cpu/common.c
@@ -593,6 +593,14 @@ void __init early_cpu_init(void)
 #endif
 }
 
+/* Make sure %gs is initialized properly in idle threads */
+struct pt_regs * __devinit idle_regs(struct pt_regs *regs)
+{
+        memset(regs, 0, sizeof(struct pt_regs));
+        regs->xgs = __KERNEL_PDA;
+        return regs;
+}
+
 __cpuinit int alloc_gdt(int cpu)
 {
         struct Xgt_desc_struct *cpu_gdt_descr = &per_cpu(cpu_gdt_descr, cpu);
@@ -644,6 +652,14 @@ struct i386_pda boot_pda = {
         ._pda = &boot_pda,
 };
 
+static inline void set_kernel_gs(void)
+{
+        /* Set %gs for this CPU's PDA.  Memory clobber is to create a
+           barrier with respect to any PDA operations, so the compiler
+           doesn't move any before here. */
+        asm volatile ("mov %0, %%gs" : : "r" (__KERNEL_PDA) : "memory");
+}
+
 /* Initialize the CPU's GDT and PDA.  The boot CPU does this for
    itself, but secondaries find this done for them. */
 __cpuinit int init_gdt(int cpu, struct task_struct *idle)
@@ -693,6 +709,7 @@ static void __cpuinit _cpu_init(int cpu, struct task_struct *curr)
            the boot CPU, this will transition from the boot gdt+pda to
            the real ones). */
         load_gdt(cpu_gdt_descr);
+        set_kernel_gs();
 
         if (cpu_test_and_set(cpu, cpu_initialized)) {
                 printk(KERN_WARNING "CPU#%d already initialized!\n", cpu);
@@ -731,8 +748,8 @@ static void __cpuinit _cpu_init(int cpu, struct task_struct *curr)
         __set_tss_desc(cpu, GDT_ENTRY_DOUBLEFAULT_TSS, &doublefault_tss);
 #endif
 
-        /* Clear %fs and %gs. */
-        asm volatile ("movl %0, %%fs; movl %0, %%gs" : : "r" (0));
+        /* Clear %fs. */
+        asm volatile ("mov %0, %%fs" : : "r" (0));
 
         /* Clear all 6 debug registers: */
         set_debugreg(0, 0);
diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S
index 0069bf01603e..b99d4a160078 100644
--- a/arch/i386/kernel/entry.S
+++ b/arch/i386/kernel/entry.S
@@ -30,12 +30,13 @@
  *      18(%esp) - %eax
  *      1C(%esp) - %ds
  *      20(%esp) - %es
- *      24(%esp) - orig_eax
- *      28(%esp) - %eip
- *      2C(%esp) - %cs
- *      30(%esp) - %eflags
- *      34(%esp) - %oldesp
- *      38(%esp) - %oldss
+ *      24(%esp) - %gs
+ *      28(%esp) - orig_eax
+ *      2C(%esp) - %eip
+ *      30(%esp) - %cs
+ *      34(%esp) - %eflags
+ *      38(%esp) - %oldesp
+ *      3C(%esp) - %oldss
  *
  * "current" is in register %ebx during any slow entries.
  */
@@ -92,6 +93,9 @@ VM_MASK		= 0x00020000
 
 #define SAVE_ALL \
         cld; \
+        pushl %gs; \
+        CFI_ADJUST_CFA_OFFSET 4;\
+        /*CFI_REL_OFFSET gs, 0;*/\
         pushl %es; \
         CFI_ADJUST_CFA_OFFSET 4;\
         /*CFI_REL_OFFSET es, 0;*/\
@@ -121,7 +125,9 @@ VM_MASK		= 0x00020000
         CFI_REL_OFFSET ebx, 0;\
         movl $(__USER_DS), %edx; \
         movl %edx, %ds; \
-        movl %edx, %es;
+        movl %edx, %es; \
+        movl $(__KERNEL_PDA), %edx; \
+        movl %edx, %gs
 
 #define RESTORE_INT_REGS \
         popl %ebx;      \
@@ -154,17 +160,22 @@ VM_MASK		= 0x00020000
 2:      popl %es;       \
         CFI_ADJUST_CFA_OFFSET -4;\
         /*CFI_RESTORE es;*/\
-.section .fixup,"ax";   \
-3:      movl $0,(%esp); \
-        jmp 1b;         \
+3:      popl %gs;       \
+        CFI_ADJUST_CFA_OFFSET -4;\
+        /*CFI_RESTORE gs;*/\
+.pushsection .fixup,"ax";       \
 4:      movl $0,(%esp); \
+        jmp 1b;         \
+5:      movl $0,(%esp); \
         jmp 2b;         \
-.previous;              \
+6:      movl $0,(%esp); \
+        jmp 3b;         \
 .section __ex_table,"a";\
         .align 4;       \
-        .long 1b,3b;    \
-        .long 2b,4b;    \
-.previous
+        .long 1b,4b;    \
+        .long 2b,5b;    \
+        .long 3b,6b;    \
+.popsection
 
 #define RING0_INT_FRAME \
         CFI_STARTPROC simple;\
@@ -231,6 +242,7 @@ check_userspace:
         andl $(VM_MASK | SEGMENT_RPL_MASK), %eax
         cmpl $USER_RPL, %eax
         jb resume_kernel                # not returning to v8086 or userspace
+
 ENTRY(resume_userspace)
         DISABLE_INTERRUPTS              # make sure we don't miss an interrupt
                                         # setting need_resched or sigpending
@@ -327,9 +339,16 @@ sysenter_past_esp:
         movl PT_OLDESP(%esp), %ecx
         xorl %ebp,%ebp
         TRACE_IRQS_ON
+1:      mov  PT_GS(%esp), %gs
         ENABLE_INTERRUPTS_SYSEXIT
         CFI_ENDPROC
-
+.pushsection .fixup,"ax"
+2:      movl $0,PT_GS(%esp)
+        jmp 1b
+.section __ex_table,"a"
+        .align 4
+        .long 1b,2b
+.popsection
 
         # system call handler stub
 ENTRY(system_call)
@@ -375,7 +394,7 @@ restore_nocheck:
         TRACE_IRQS_IRET
 restore_nocheck_notrace:
         RESTORE_REGS
-        addl $4, %esp
+        addl $4, %esp                   # skip orig_eax/error_code
         CFI_ADJUST_CFA_OFFSET -4
 1:      INTERRUPT_RETURN
 .section .fixup,"ax"
@@ -588,6 +607,10 @@ KPROBE_ENTRY(page_fault)
         CFI_ADJUST_CFA_OFFSET 4
         ALIGN
 error_code:
+        /* the function address is in %gs's slot on the stack */
+        pushl %es
+        CFI_ADJUST_CFA_OFFSET 4
+        /*CFI_REL_OFFSET es, 0*/
         pushl %ds
         CFI_ADJUST_CFA_OFFSET 4
         /*CFI_REL_OFFSET ds, 0*/
@@ -613,18 +636,20 @@ error_code:
         CFI_ADJUST_CFA_OFFSET 4
         CFI_REL_OFFSET ebx, 0
         cld
-        pushl %es
+        pushl %gs
         CFI_ADJUST_CFA_OFFSET 4
-        /*CFI_REL_OFFSET es, 0*/
+        /*CFI_REL_OFFSET gs, 0*/
+        movl $(__KERNEL_PDA), %ecx
+        movl %ecx, %gs
         UNWIND_ESPFIX_STACK
         popl %ecx
         CFI_ADJUST_CFA_OFFSET -4
         /*CFI_REGISTER es, ecx*/
-        movl PT_ES(%esp), %edi          # get the function address
+        movl PT_GS(%esp), %edi          # get the function address
         movl PT_ORIG_EAX(%esp), %edx    # get the error code
-        movl $-1, PT_ORIG_EAX(%esp)
-        movl %ecx, PT_ES(%esp)
-        /*CFI_REL_OFFSET es, ES*/
+        movl $-1, PT_ORIG_EAX(%esp)     # no syscall to restart
+        mov  %ecx, PT_GS(%esp)
+        /*CFI_REL_OFFSET gs, ES*/
         movl $(__USER_DS), %ecx
         movl %ecx, %ds
         movl %ecx, %es
@@ -936,6 +961,7 @@ ENTRY(arch_unwind_init_running)
         movl    %ebx, PT_EAX(%edx)
         movl    $__USER_DS, PT_DS(%edx)
         movl    $__USER_DS, PT_ES(%edx)
+        movl    $0, PT_GS(%edx)
         movl    %ebx, PT_ORIG_EAX(%edx)
         movl    %ecx, PT_EIP(%edx)
         movl    12(%esp), %ecx
diff --git a/arch/i386/kernel/head.S b/arch/i386/kernel/head.S
index 4a83384c5a61..5b14e95ac8b9 100644
--- a/arch/i386/kernel/head.S
+++ b/arch/i386/kernel/head.S
@@ -302,6 +302,7 @@ is386:	movl $2,%ecx		# set MP
         movl %eax,%cr0
 
         call check_x87
+        call setup_pda
         lgdt cpu_gdt_descr
         lidt idt_descr
         ljmp $(__KERNEL_CS),$1f
@@ -312,10 +313,13 @@ is386:	movl $2,%ecx		# set MP
         movl %eax,%ds
         movl %eax,%es
 
-        xorl %eax,%eax                  # Clear FS/GS and LDT
+        xorl %eax,%eax                  # Clear FS and LDT
         movl %eax,%fs
-        movl %eax,%gs
         lldt %ax
+
+        movl $(__KERNEL_PDA),%eax
+        mov  %eax,%gs
+
         cld                     # gcc2 wants the direction flag cleared at all times
         pushl $0                # fake return address for unwinder
 #ifdef CONFIG_SMP
@@ -346,6 +350,23 @@ check_x87:
         ret
 
 /*
+ * Point the GDT at this CPU's PDA.  On boot this will be
+ * cpu_gdt_table and boot_pda; for secondary CPUs, these will be
+ * that CPU's GDT and PDA.
+ */
+setup_pda:
+        /* get the PDA pointer */
+        movl start_pda, %eax
+
+        /* slot the PDA address into the GDT */
+        mov cpu_gdt_descr+2, %ecx
+        mov %ax, (__KERNEL_PDA+0+2)(%ecx)               /* base & 0x0000ffff */
+        shr $16, %eax
+        mov %al, (__KERNEL_PDA+4+0)(%ecx)               /* base & 0x00ff0000 */
+        mov %ah, (__KERNEL_PDA+4+3)(%ecx)               /* base & 0xff000000 */
+        ret
+
+/*
  *  setup_idt
  *
  *  sets up a idt with 256 entries pointing to
@@ -484,6 +505,8 @@ ENTRY(empty_zero_page)
  * This starts the data section.
  */
 .data
+ENTRY(start_pda)
+        .long boot_pda
 
 ENTRY(stack_start)
         .long init_thread_union+THREAD_SIZE
@@ -525,7 +548,7 @@ idt_descr:
 
 # boot GDT descriptor (later on used by CPU#0):
         .word 0                         # 32 bit align gdt_desc.address
-cpu_gdt_descr:
+ENTRY(cpu_gdt_descr)
         .word GDT_ENTRIES*8-1
         .long cpu_gdt_table
 
@@ -585,7 +608,7 @@ ENTRY(cpu_gdt_table)
         .quad 0x004092000000ffff        /* 0xc8 APM DS    data */
 
         .quad 0x00c0920000000000        /* 0xd0 - ESPFIX SS */
-        .quad 0x0000000000000000        /* 0xd8 - PDA */
+        .quad 0x00cf92000000ffff        /* 0xd8 - PDA */
         .quad 0x0000000000000000        /* 0xe0 - unused */
         .quad 0x0000000000000000        /* 0xe8 - unused */
         .quad 0x0000000000000000        /* 0xf0 - unused */
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.
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);