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Subject[PATCH v2 7/8] x86/segments/32: Introduce CPU_NUMBER segment
Date
The new entry will be equivalent to that of x86-64 which
stores CPU number. The entry is placed in segment 23 in GDT
by bumping down 23-28 by one, which are all kernel-internal
segments and so have no impact on user space.

CPU_NUMBER segment will always be at '%ss (USER_DS) + 80'
for the default (flat, initial) user space %ss. %ss is
specified than %ds because it is less likely to be changed
as 64-bit has %ss defined.

Suggested-by: H. Peter <hpa@zytor.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.org>
---
arch/x86/include/asm/segment.h | 30 +++++++++++++++++++++---------
1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 492e3d1..fca55d7 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -77,14 +77,14 @@
* 20 - PNPBIOS support <=== cacheline #6
* 21 - PNPBIOS support
* 22 - PNPBIOS support
- * 23 - APM BIOS support
+ * 23 - CPU number
* 24 - APM BIOS support <=== cacheline #7
* 25 - APM BIOS support
*
- * 26 - ESPFIX small SS
- * 27 - per-cpu [ offset to per-cpu data area ]
- * 28 - stack_canary-20 [ for stack protector ] <=== cacheline #8
- * 29 - unused
+ * 26 - APM BIOS support
+ * 27 - ESPFIX small SS
+ * 28 - per-cpu [ offset to per-cpu data area ] <=== cacheline #8
+ * 29 - stack_canary-20 [ for stack protector ]
* 30 - unused
* 31 - TSS for double fault handler
*/
@@ -102,11 +102,12 @@
#define GDT_ENTRY_PNPBIOS_DS 20
#define GDT_ENTRY_PNPBIOS_TS1 21
#define GDT_ENTRY_PNPBIOS_TS2 22
-#define GDT_ENTRY_APMBIOS_BASE 23
+#define GDT_ENTRY_CPU_NUMBER 23
+#define GDT_ENTRY_APMBIOS_BASE 24

-#define GDT_ENTRY_ESPFIX_SS 26
-#define GDT_ENTRY_PERCPU 27
-#define GDT_ENTRY_STACK_CANARY 28
+#define GDT_ENTRY_ESPFIX_SS 27
+#define GDT_ENTRY_PERCPU 28
+#define GDT_ENTRY_STACK_CANARY 29

#define GDT_ENTRY_DOUBLEFAULT_TSS 31

@@ -140,6 +141,13 @@
/* another data segment: */
#define PNP_TS2 (GDT_ENTRY_PNPBIOS_TS2*8)

+/*
+ * CPU_NUMBER segment is at '%ss + 80' for the default (flat, initial)
+ * user space %ss (for 64-bit as well). Using %ss than %ds for less
+ * likely to be changed as defined in 64-bit too.
+ */
+#define __CPU_NUMBER_SEG (GDT_ENTRY_CPU_NUMBER*8 + 3)
+
#ifdef CONFIG_SMP
# define __KERNEL_PERCPU (GDT_ENTRY_PERCPU*8)
#else
@@ -206,6 +214,10 @@
#define __USER_DS (GDT_ENTRY_DEFAULT_USER_DS*8 + 3)
#define __USER32_DS __USER_DS
#define __USER_CS (GDT_ENTRY_DEFAULT_USER_CS*8 + 3)
+/*
+ * CPU_NUMBER segment at '%ss (USER_DS) + 80',
+ * like 32-bit for the same reason
+ */
#define __CPU_NUMBER_SEG (GDT_ENTRY_CPU_NUMBER*8 + 3)

#endif
--
2.7.4
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