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Subject[RFC PATCH 17/35] Segment register changes for Xen
1. We clear FS/GS before changing TLS entries and switching LDT, as
otherwise the hypervisor will fail to restore thread-local values on
return to the guest kernel and we take a slow exception path.

2. We allow for the fact that the guest kernel may not run in ring 0.
This requires some abstraction in a few places when setting %cs or
checking privilege level (user vs kernel).

Signed-off-by: Ian Pratt <ian.pratt@xensource.com>
Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
---
arch/i386/kernel/process.c | 4 +++-
arch/i386/mm/fault.c | 8 +++++---
include/asm-i386/mach-default/mach_segment.h | 8 ++++++++
include/asm-i386/mach-default/mach_system.h | 2 ++
include/asm-i386/mach-xen/mach_segment.h | 9 +++++++++
include/asm-i386/mach-xen/mach_system.h | 2 ++
include/asm-i386/mach-xen/setup_arch_post.h | 2 ++
include/asm-i386/ptrace.h | 6 ++++--
include/asm-i386/segment.h | 2 ++
9 files changed, 37 insertions(+), 6 deletions(-)

--- xen-subarch-2.6.orig/arch/i386/kernel/process.c
+++ xen-subarch-2.6/arch/i386/kernel/process.c
@@ -348,7 +348,7 @@ int kernel_thread(int (*fn)(void *), voi
regs.xes = __USER_DS;
regs.orig_eax = -1;
regs.eip = (unsigned long) kernel_thread_helper;
- regs.xcs = __KERNEL_CS;
+ regs.xcs = get_kernel_cs();
regs.eflags = X86_EFLAGS_IF | X86_EFLAGS_SF | X86_EFLAGS_PF | 0x2;

/* Ok, create the new process.. */
@@ -655,6 +655,8 @@ struct task_struct fastcall * __switch_t
*/
savesegment(fs, prev->fs);
savesegment(gs, prev->gs);
+ clearsegment(fs);
+ clearsegment(gs);

/*
* Load the per-thread Thread-Local Storage descriptor.
--- xen-subarch-2.6.orig/arch/i386/mm/fault.c
+++ xen-subarch-2.6/arch/i386/mm/fault.c
@@ -28,6 +28,8 @@
#include <asm/desc.h>
#include <asm/kdebug.h>

+#include <mach_segment.h>
+
extern void die(const char *,struct pt_regs *,long);

/*
@@ -78,14 +80,14 @@ static inline unsigned long get_segment_
u32 seg_ar, seg_limit, base, *desc;

/* The standard kernel/user address space limit. */
- *eip_limit = (seg & 3) ? USER_DS.seg : KERNEL_DS.seg;
+ *eip_limit = (seg & USER_MODE_MASK) ? USER_DS.seg : KERNEL_DS.seg;

/* Unlikely, but must come before segment checks. */
if (unlikely((regs->eflags & VM_MASK) != 0))
return eip + (seg << 4);

/* By far the most common cases. */
- if (likely(seg == __USER_CS || seg == __KERNEL_CS))
+ if (likely(seg == __USER_CS || seg == get_kernel_cs()))
return eip;

/* Check the segment exists, is within the current LDT/GDT size,
@@ -331,7 +333,7 @@ good_area:
switch (error_code & 3) {
default: /* 3: write, present */
#ifdef TEST_VERIFY_AREA
- if (regs->cs == KERNEL_CS)
+ if (regs->cs == get_kernel_cs())
printk("WP fault at %08lx\n", regs->eip);
#endif
/* fall through */
--- xen-subarch-2.6.orig/include/asm-i386/mach-default/mach_system.h
+++ xen-subarch-2.6/include/asm-i386/mach-default/mach_system.h
@@ -1,6 +1,8 @@
#ifndef __ASM_MACH_SYSTEM_H
#define __ASM_MACH_SYSTEM_H

+#define clearsegment(seg)
+
/* interrupt control.. */
#define local_save_flags(x) do { typecheck(unsigned long,x); __asm__ __volatile__("pushfl ; popl %0":"=g" (x): /* no input */); } while (0)
#define local_irq_restore(x) do { typecheck(unsigned long,x); __asm__ __volatile__("pushl %0 ; popfl": /* no output */ :"g" (x):"memory", "cc"); } while (0)
--- xen-subarch-2.6.orig/include/asm-i386/mach-xen/mach_system.h
+++ xen-subarch-2.6/include/asm-i386/mach-xen/mach_system.h
@@ -5,6 +5,8 @@

#include <asm/hypervisor.h>

+#define clearsegment(seg) loadsegment(seg, 0)
+
#ifdef CONFIG_SMP
#define __vcpu_id smp_processor_id()
#else
--- xen-subarch-2.6.orig/include/asm-i386/mach-xen/setup_arch_post.h
+++ xen-subarch-2.6/include/asm-i386/mach-xen/setup_arch_post.h
@@ -39,6 +39,8 @@ static void __init machine_specific_arch
(struct shared_info *)__va(xen_start_info->shared_info);
memset(empty_zero_page, 0, sizeof(empty_zero_page));

+ HYPERVISOR_vm_assist(VMASST_CMD_enable, VMASST_TYPE_4gb_segments);
+
HYPERVISOR_set_callbacks(
__KERNEL_CS, (unsigned long)hypervisor_callback,
__KERNEL_CS, (unsigned long)failsafe_callback);
--- xen-subarch-2.6.orig/include/asm-i386/ptrace.h
+++ xen-subarch-2.6/include/asm-i386/ptrace.h
@@ -1,6 +1,8 @@
#ifndef _I386_PTRACE_H
#define _I386_PTRACE_H

+#include <mach_segment.h>
+
#define EBX 0
#define ECX 1
#define EDX 2
@@ -73,11 +75,11 @@ extern void send_sigtrap(struct task_str
*/
static inline int user_mode(struct pt_regs *regs)
{
- return (regs->xcs & 3) != 0;
+ return (regs->xcs & USER_MODE_MASK) != 0;
}
static inline int user_mode_vm(struct pt_regs *regs)
{
- return ((regs->xcs & 3) | (regs->eflags & VM_MASK)) != 0;
+ return ((regs->xcs & USER_MODE_MASK) | (regs->eflags & VM_MASK)) != 0;
}
#define instruction_pointer(regs) ((regs)->eip)
#if defined(CONFIG_SMP) && defined(CONFIG_FRAME_POINTER)
--- xen-subarch-2.6.orig/include/asm-i386/segment.h
+++ xen-subarch-2.6/include/asm-i386/segment.h
@@ -1,6 +1,8 @@
#ifndef _ASM_SEGMENT_H
#define _ASM_SEGMENT_H

+#include <mach_segment.h>
+
/*
* The layout of the per-CPU GDT under Linux:
*
--- /dev/null
+++ xen-subarch-2.6/include/asm-i386/mach-default/mach_segment.h
@@ -0,0 +1,8 @@
+#ifndef __ASM_MACH_SEGMENT_H
+#define __ASM_MACH_SEGMENT_H
+
+#define USER_MODE_MASK 3
+
+#define get_kernel_cs() __KERNEL_CS
+
+#endif /* __ASM_MACH_SEGMENT_H */
--- /dev/null
+++ xen-subarch-2.6/include/asm-i386/mach-xen/mach_segment.h
@@ -0,0 +1,9 @@
+#ifndef __ASM_MACH_SEGMENT_H
+#define __ASM_MACH_SEGMENT_H
+
+#define USER_MODE_MASK 2
+
+#define get_kernel_cs() \
+ (__KERNEL_CS + (xen_feature(XENFEAT_supervisor_mode_kernel) ? 0 : 1))
+
+#endif /* __ASM_MACH_SEGMENT_H */
--
-
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