WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-devel

[Xen-devel] [RFC PATCH 17/35] Segment register changes for Xen

To: linux-kernel@xxxxxxxxxxxxxxx
Subject: [Xen-devel] [RFC PATCH 17/35] Segment register changes for Xen
From: Chris Wright <chrisw@xxxxxxxxxxxx>
Date: Tue, 21 Mar 2006 22:30:57 -0800
Cc: virtualization@xxxxxxxxxxxxxx, Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, Ian Pratt <ian.pratt@xxxxxxxxxxxxx>
Delivery-date: Wed, 22 Mar 2006 06:56:16 +0000
Envelope-to: www-data@xxxxxxxxxxxxxxxxxxx
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
References: <20060322063040.960068000@xxxxxxxxxxxxxxxxxx>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
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@xxxxxxxxxxxxx>
Signed-off-by: Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>
Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
---
 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 */

--

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel

<Prev in Thread] Current Thread [Next in Thread>