[PATCH] i386: Allow a kernel not to be in ring 0

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).

This is Chris' [RFC PATCH 15/33] move segment checks to subarch, except rather
than using #define USER_MODE_MASK which depends on a config option, we use
Zach's more flexible approach of assuming ring 3 == userspace.  I also used
"get_kernel_rpl()" over "get_kernel_cs()" because I think it reads better in
the code...

1) Remove the hardcoded 3 and introduce #define SEGMENT_RPL_MASK 3 2) Add a
get_kernel_rpl() macro, and don't assume it's zero.

And:

Clean up of patch for letting kernel run other than ring 0:

a. Add some comments about the SEGMENT_IS_*_CODE() macros.
b. Add a USER_RPL macro.  (Code was comparing a value to a mask
   in some places and to the magic number 3 in other places.)
c. Add macros for table indicator field and use them.
d. Change the entry.S tests for LDT stack segment to use the macros

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Zachary Amsden <zach@vmware.com>
Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Andi Kleen <ak@suse.de>

2 files changed, 5 insertions, 8 deletions

diff --git a/arch/i386/mm/extable.c b/arch/i386/mm/extable.c
index de03c5430abc..0ce4f22a2635 100644
--- a/arch/i386/mm/extable.c
+++ b/arch/i386/mm/extable.c
@@ -11,7 +11,7 @@ int fixup_exception(struct pt_regs *regs)
         const struct exception_table_entry *fixup;
 
 #ifdef CONFIG_PNPBIOS
-        if (unlikely((regs->xcs & ~15) == (GDT_ENTRY_PNPBIOS_BASE << 3)))
+        if (unlikely(SEGMENT_IS_PNP_CODE(regs->xcs)))
         {
                 extern u32 pnp_bios_fault_eip, pnp_bios_fault_esp;
                 extern u32 pnp_bios_is_utter_crap;
diff --git a/arch/i386/mm/fault.c b/arch/i386/mm/fault.c
index 0ce86168a0b1..5e17a3f43b41 100644
--- a/arch/i386/mm/fault.c
+++ b/arch/i386/mm/fault.c
@@ -27,6 +27,7 @@
 #include <asm/uaccess.h>
 #include <asm/desc.h>
 #include <asm/kdebug.h>
+#include <asm/segment.h>
 
 extern void die(const char *,struct pt_regs *,long);
 
@@ -113,10 +114,10 @@ static inline unsigned long get_segment_eip(struct pt_regs *regs,
         }
 
         /* The standard kernel/user address space limit. */
-        *eip_limit = (seg & 3) ? USER_DS.seg : KERNEL_DS.seg;
+        *eip_limit = user_mode(regs) ? USER_DS.seg : KERNEL_DS.seg;
         
         /* By far the most common cases. */
-        if (likely(seg == __USER_CS || seg == __KERNEL_CS))
+        if (likely(SEGMENT_IS_FLAT_CODE(seg)))
                 return eip;
 
         /* Check the segment exists, is within the current LDT/GDT size,
@@ -430,11 +431,7 @@ good_area:
         write = 0;
         switch (error_code & 3) {
                 default:        /* 3: write, present */
-#ifdef TEST_VERIFY_AREA
-                        if (regs->cs == KERNEL_CS)
-                                printk("WP fault at %08lx\n", regs->eip);
-#endif
-                        /* fall through */
+                                /* fall through */
                 case 2:         /* write, not present */
                         if (!(vma->vm_flags & VM_WRITE))
                                 goto bad_area;