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Subject[PATCH 2/7]
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This patch implements save/restore of %gs in the kernel, so it can be
used for per-cpu data. This is not cheap, and we do it for UP as well
as SMP, which is stupid. Benchmarks, anyone?

Based on this patch by Jeremy Fitzhardinge:

From: Jeremy Fitzhardinge <jeremy@goop.org>
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.

NOTE: even though it's called "ptrace-abi.h", this file does not
define a user-space visible ABI.

And

+From: Ingo Molnar <mingo@elte.hu>

in the syscall exit path the %gs selector has to be restored _after_ the
last kernel function has been called. If lockdep is enabled then this
kernel function is TRACE_IRQS_ON.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
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: Rusty Russell <rusty@rustcorp.com.au>

Index: ak-pda-gs/arch/i386/kernel/asm-offsets.c
===================================================================
--- ak-pda-gs.orig/arch/i386/kernel/asm-offsets.c 2006-09-20 16:40:06.000000000 +1000
+++ ak-pda-gs/arch/i386/kernel/asm-offsets.c 2006-09-20 16:40:18.000000000 +1000
@@ -67,6 +67,7 @@
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);
Index: ak-pda-gs/arch/i386/kernel/cpu/common.c
===================================================================
--- ak-pda-gs.orig/arch/i386/kernel/cpu/common.c 2006-09-20 16:40:06.000000000 +1000
+++ ak-pda-gs/arch/i386/kernel/cpu/common.c 2006-09-20 16:40:18.000000000 +1000
@@ -579,6 +579,15 @@
disable_pse = 1;
#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_PERCPU;
+ return regs;
+}
+
/*
* cpu_init() initializes state that is per-CPU. Some data is already
* initialized (naturally) in the bootstrap process, such as the GDT
@@ -641,8 +650,8 @@
__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);
Index: ak-pda-gs/arch/i386/kernel/entry.S
===================================================================
--- ak-pda-gs.orig/arch/i386/kernel/entry.S 2006-09-20 16:40:06.000000000 +1000
+++ ak-pda-gs/arch/i386/kernel/entry.S 2006-09-20 16:40:18.000000000 +1000
@@ -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.
*/
@@ -91,6 +92,9 @@

#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;*/\
@@ -120,7 +124,9 @@
CFI_REL_OFFSET ebx, 0;\
movl $(__USER_DS), %edx; \
movl %edx, %ds; \
- movl %edx, %es;
+ movl %edx, %es; \
+ movl $(__KERNEL_PERCPU), %edx; \
+ movl %edx, %gs

#define RESTORE_INT_REGS \
popl %ebx; \
@@ -153,17 +159,22 @@
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;\
@@ -320,11 +331,18 @@
/* if something modifies registers it must also disable sysexit */
movl PT_EIP(%esp), %edx
movl PT_OLDESP(%esp), %ecx
- xorl %ebp,%ebp
TRACE_IRQS_ON
+1: mov PT_GS(%esp), %gs
+ xorl %ebp,%ebp
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)
@@ -370,7 +388,7 @@
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"
@@ -512,14 +530,12 @@
/* put ESP to the proper location */ \
movl %eax, %esp;
#define UNWIND_ESPFIX_STACK \
- pushl %eax; \
CFI_ADJUST_CFA_OFFSET 4; \
movl %ss, %eax; \
/* see if on 16bit stack */ \
- cmpw $__ESPFIX_SS, %ax; \
+ cmp $__ESPFIX_SS, %eax; \
je 28f; \
-27: popl %eax; \
- CFI_ADJUST_CFA_OFFSET -4; \
+27: CFI_ADJUST_CFA_OFFSET -4; \
.section .fixup,"ax"; \
28: movl $__KERNEL_DS, %eax; \
movl %eax, %ds; \
@@ -588,13 +604,15 @@
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
pushl %ds
CFI_ADJUST_CFA_OFFSET 4
/*CFI_REL_OFFSET ds, 0*/
pushl %eax
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET eax, 0
- xorl %eax, %eax
pushl %ebp
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET ebp, 0
@@ -607,7 +625,6 @@
pushl %edx
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET edx, 0
- decl %eax # eax = -1
pushl %ecx
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET ecx, 0
@@ -615,21 +632,17 @@
CFI_ADJUST_CFA_OFFSET 4
CFI_REL_OFFSET ebx, 0
cld
- pushl %es
- CFI_ADJUST_CFA_OFFSET 4
- /*CFI_REL_OFFSET es, 0*/
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 %eax, 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 %gs, PT_GS(%esp)
+ /*CFI_REL_OFFSET gs, GS*/
movl $(__USER_DS), %ecx
movl %ecx, %ds
movl %ecx, %es
+ movl $(__KERNEL_PERCPU), %ecx
+ movl %ecx, %gs
movl %esp,%eax # pt_regs pointer
call *%edi
jmp ret_from_exception
@@ -939,6 +952,7 @@
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
Index: ak-pda-gs/arch/i386/kernel/process.c
===================================================================
--- ak-pda-gs.orig/arch/i386/kernel/process.c 2006-09-20 16:40:06.000000000 +1000
+++ ak-pda-gs/arch/i386/kernel/process.c 2006-09-20 16:40:18.000000000 +1000
@@ -336,6 +336,7 @@

regs.xds = __USER_DS;
regs.xes = __USER_DS;
+ regs.xgs = __KERNEL_PERCPU;
regs.orig_eax = -1;
regs.eip = (unsigned long) kernel_thread_helper;
regs.xcs = __KERNEL_CS | get_kernel_rpl();
@@ -421,7 +422,6 @@
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))) {
@@ -645,16 +645,16 @@
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.
@@ -662,16 +662,13 @@
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.
Index: ak-pda-gs/arch/i386/kernel/signal.c
===================================================================
--- ak-pda-gs.orig/arch/i386/kernel/signal.c 2006-09-20 16:40:06.000000000 +1000
+++ ak-pda-gs/arch/i386/kernel/signal.c 2006-09-20 16:40:18.000000000 +1000
@@ -128,7 +128,7 @@
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 @@
{
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);

Index: ak-pda-gs/include/asm-i386/mmu_context.h
===================================================================
--- ak-pda-gs.orig/include/asm-i386/mmu_context.h 2006-09-20 16:40:06.000000000 +1000
+++ ak-pda-gs/include/asm-i386/mmu_context.h 2006-09-20 16:40:18.000000000 +1000
@@ -62,8 +62,8 @@
#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)
Index: ak-pda-gs/include/asm-i386/processor.h
===================================================================
--- ak-pda-gs.orig/include/asm-i386/processor.h 2006-09-20 16:40:06.000000000 +1000
+++ ak-pda-gs/include/asm-i386/processor.h 2006-09-20 16:40:18.000000000 +1000
@@ -477,6 +477,7 @@
.vm86_info = NULL, \
.sysenter_cs = __KERNEL_CS, \
.io_bitmap_ptr = NULL, \
+ .gs = __KERNEL_PERCPU, \
}

/*
@@ -504,7 +505,8 @@
}

#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; \
Index: ak-pda-gs/kernel/fork.c
===================================================================
--- ak-pda-gs.orig/kernel/fork.c 2006-09-20 16:40:06.000000000 +1000
+++ ak-pda-gs/kernel/fork.c 2006-09-20 16:40:18.000000000 +1000
@@ -1299,7 +1299,7 @@
return ERR_PTR(retval);
}

-struct pt_regs * __devinit __attribute__((weak)) idle_regs(struct pt_regs *regs)
+noinline struct pt_regs * __devinit __attribute__((weak)) idle_regs(struct pt_regs *regs)
{
memset(regs, 0, sizeof(struct pt_regs));
return regs;
Index: ak-pda-gs/include/asm-i386/ptrace.h
===================================================================
--- ak-pda-gs.orig/include/asm-i386/ptrace.h 2006-09-20 16:40:06.000000000 +1000
+++ ak-pda-gs/include/asm-i386/ptrace.h 2006-09-20 16:40:18.000000000 +1000
@@ -33,6 +33,8 @@
long eax;
int xds;
int xes;
+ /* int xfs; */
+ int xgs;
long orig_eax;
long eip;
int xcs;
Index: ak-pda-gs/include/asm-i386/segment.h
===================================================================
--- ak-pda-gs.orig/include/asm-i386/segment.h 2006-09-20 16:40:01.000000000 +1000
+++ ak-pda-gs/include/asm-i386/segment.h 2006-09-20 16:40:18.000000000 +1000
@@ -39,7 +39,7 @@
* 25 - APM BIOS support
*
* 26 - ESPFIX small SS
- * 27 - unused
+ * 27 - PERCPU [ offset segment for per-cpu area ]
* 28 - unused
* 29 - unused
* 30 - unused
@@ -74,6 +74,9 @@
#define GDT_ENTRY_ESPFIX_SS (GDT_ENTRY_KERNEL_BASE + 14)
#define __ESPFIX_SS (GDT_ENTRY_ESPFIX_SS * 8)

+#define GDT_ENTRY_PERCPU (GDT_ENTRY_KERNEL_BASE + 15)
+#define __KERNEL_PERCPU (GDT_ENTRY_PERCPU * 8)
+
#define GDT_ENTRY_DOUBLEFAULT_TSS 31

/*
Index: ak-pda-gs/include/asm-i386/unwind.h
===================================================================
--- ak-pda-gs.orig/include/asm-i386/unwind.h 2006-09-20 16:40:01.000000000 +1000
+++ ak-pda-gs/include/asm-i386/unwind.h 2006-09-20 16:40:18.000000000 +1000
@@ -64,6 +64,7 @@
info->regs.xss = __KERNEL_DS;
info->regs.xds = __USER_DS;
info->regs.xes = __USER_DS;
+ info->regs.xgs = __KERNEL_PERCPU;
}

extern asmlinkage int arch_unwind_init_running(struct unwind_frame_info *,
--
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