linux-kernel - [PATCH 1/3] x86: Move TSS and LDT to end of the GDT

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Date:   Wed, 13 Dec 2023 11:34:41 -0500
From:   Brian Gerst <brgerst@...il.com>
To:     linux-kernel@...r.kernel.org, x86@...nel.org
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>,
        Brian Gerst <brgerst@...il.com>
Subject: [PATCH 1/3] x86: Move TSS and LDT to end of the GDT

This will make testing for system segments easier.

Signed-off-by: Brian Gerst <brgerst@...il.com>
---
 arch/x86/include/asm/segment.h | 33 +++++++++++++++++++++------------
 1 file changed, 21 insertions(+), 12 deletions(-)

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 9d6411c65920..a155843d0c37 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -83,8 +83,8 @@
  *  13 - kernel data segment
  *  14 - default user CS
  *  15 - default user DS
- *  16 - TSS								<=== cacheline #5
- *  17 - LDT
+ *  16 - unused								<=== cacheline #5
+ *  17 - unused
  *  18 - PNPBIOS support (16->32 gate)
  *  19 - PNPBIOS support
  *  20 - PNPBIOS support						<=== cacheline #6
@@ -97,8 +97,11 @@
  *  26 - ESPFIX small SS
  *  27 - per-cpu			[ offset to per-cpu data area ]
  *  28 - VDSO getcpu
- *  29 - unused
- *  30 - unused
+ *
+ *  ------- start of system segments:
+ *
+ *  29 - TSS
+ *  30 - LDT
  *  31 - TSS for double fault handler
  */
 #define GDT_ENTRY_TLS_MIN		6
@@ -108,8 +111,6 @@
 #define GDT_ENTRY_KERNEL_DS		13
 #define GDT_ENTRY_DEFAULT_USER_CS	14
 #define GDT_ENTRY_DEFAULT_USER_DS	15
-#define GDT_ENTRY_TSS			16
-#define GDT_ENTRY_LDT			17
 #define GDT_ENTRY_PNPBIOS_CS32		18
 #define GDT_ENTRY_PNPBIOS_CS16		19
 #define GDT_ENTRY_PNPBIOS_DS		20
@@ -121,6 +122,10 @@
 #define GDT_ENTRY_PERCPU		27
 #define GDT_ENTRY_CPUNODE		28
 
+/* Start of system segments */
+
+#define GDT_ENTRY_TSS			29
+#define GDT_ENTRY_LDT			30
 #define GDT_ENTRY_DOUBLEFAULT_TSS	31
 
 /*
@@ -188,20 +193,22 @@
 #define GDT_ENTRY_DEFAULT_USER_DS	5
 #define GDT_ENTRY_DEFAULT_USER_CS	6
 
-/* Needs two entries */
-#define GDT_ENTRY_TSS			8
-/* Needs two entries */
-#define GDT_ENTRY_LDT			10
-
 #define GDT_ENTRY_TLS_MIN		12
 #define GDT_ENTRY_TLS_MAX		14
 
 #define GDT_ENTRY_CPUNODE		15
 
+/* Start of system segments */
+
+/* Needs two entries */
+#define GDT_ENTRY_TSS			16
+/* Needs two entries */
+#define GDT_ENTRY_LDT			18
+
 /*
  * Number of entries in the GDT table:
  */
-#define GDT_ENTRIES			16
+#define GDT_ENTRIES			20
 
 /*
  * Segment selector values corresponding to the above entries:
@@ -219,6 +226,8 @@
 
 #endif
 
+#define GDT_SYSTEM_START		GDT_ENTRY_TSS
+
 #define IDT_ENTRIES			256
 #define NUM_EXCEPTION_VECTORS		32
 
-- 
2.43.0