x86/asm: Further improve segment.h readability

 - extend/clarify explanations where necessary

 - move comments from macro values to before the macro, to
   make them more consistent, and to reduce preprocessor overhead

 - sort GDT index and selector values likewise by number

 - use consistent, modern kernel coding style across the file

 - capitalize consistently

 - use consistent vertical spacing

 - remove the unused get_limit() method (noticed by Andy Lutomirski)

No change in code (verified with objdump -d):

 64-bit defconfig+kvmconfig:

   815a129bc1f80de6445c1d8ca5b97cad  vmlinux.o.before.asm
   815a129bc1f80de6445c1d8ca5b97cad  vmlinux.o.after.asm

 32-bit defconfig+kvmconfig:

   e659ef045159ddf41a0771b33a34aae5  vmlinux.o.before.asm
   e659ef045159ddf41a0771b33a34aae5  vmlinux.o.after.asm

Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Alexei Starovoitov <ast@plumgrid.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/segment.h

@@ -3,8 +3,10 @@
 
 #include <linux/const.h>
 
-/* Constructor for a conventional segment GDT (or LDT) entry */
-/* This is a macro so it can be used in initializers */
+/*
+ * Constructor for a conventional segment GDT (or LDT) entry.
+ * This is a macro so it can be used in initializers.
+ */
 #define GDT_ENTRY(flags, base, limit)			\
 	((((base)  & _AC(0xff000000,ULL)) << (56-24)) |	\
 	 (((flags) & _AC(0x0000f0ff,ULL)) << 40) |	\
@@ -12,71 +14,78 @@
 	 (((base)  & _AC(0x00ffffff,ULL)) << 16) |	\
 	 (((limit) & _AC(0x0000ffff,ULL))))
 
-/* Simple and small GDT entries for booting only */
+/* Simple and small GDT entries for booting only: */
 
 #define GDT_ENTRY_BOOT_CS	2
 #define GDT_ENTRY_BOOT_DS	3
 #define GDT_ENTRY_BOOT_TSS	4
-#define __BOOT_CS		(GDT_ENTRY_BOOT_CS * 8)
-#define __BOOT_DS		(GDT_ENTRY_BOOT_DS * 8)
-#define __BOOT_TSS		(GDT_ENTRY_BOOT_TSS * 8)
-
-#define SEGMENT_RPL_MASK	0x3 /*
-				     * Bottom two bits of selector give the ring
-				     * privilege level
-				     */
-#define SEGMENT_TI_MASK		0x4 /* Bit 2 is table indicator (LDT/GDT) */
-#define USER_RPL		0x3 /* User mode is privilege level 3 */
-#define SEGMENT_LDT		0x4 /* LDT segment has TI set... */
-#define SEGMENT_GDT		0x0 /* ... GDT has it cleared */
+#define __BOOT_CS		(GDT_ENTRY_BOOT_CS*8)
+#define __BOOT_DS		(GDT_ENTRY_BOOT_DS*8)
+#define __BOOT_TSS		(GDT_ENTRY_BOOT_TSS*8)
+
+/*
+ * Bottom two bits of selector give the ring
+ * privilege level
+ */
+#define SEGMENT_RPL_MASK	0x3
+
+/* User mode is privilege level 3: */
+#define USER_RPL		0x3
+
+/* Bit 2 is Table Indicator (TI): selects between LDT or GDT */
+#define SEGMENT_TI_MASK		0x4
+/* LDT segment has TI set ... */
+#define SEGMENT_LDT		0x4
+/* ... GDT has it cleared */
+#define SEGMENT_GDT		0x0
 
 #ifdef CONFIG_X86_32
 /*
  * The layout of the per-CPU GDT under Linux:
  *
- *   0 - null
+ *   0 - null								<=== cacheline #1
  *   1 - reserved
  *   2 - reserved
  *   3 - reserved
  *
- *   4 - unused			<==== new cacheline
+ *   4 - unused								<=== cacheline #2
  *   5 - unused
  *
  *  ------- start of TLS (Thread-Local Storage) segments:
  *
  *   6 - TLS segment #1			[ glibc's TLS segment ]
  *   7 - TLS segment #2			[ Wine's %fs Win32 segment ]
- *   8 - TLS segment #3
+ *   8 - TLS segment #3							<=== cacheline #3
  *   9 - reserved
  *  10 - reserved
  *  11 - reserved
  *
  *  ------- start of kernel segments:
  *
- *  12 - kernel code segment		<==== new cacheline
+ *  12 - kernel code segment						<=== cacheline #4
  *  13 - kernel data segment
  *  14 - default user CS
  *  15 - default user DS
- *  16 - TSS
+ *  16 - TSS								<=== cacheline #5
  *  17 - LDT
  *  18 - PNPBIOS support (16->32 gate)
  *  19 - PNPBIOS support
- *  20 - PNPBIOS support
+ *  20 - PNPBIOS support						<=== cacheline #6
  *  21 - PNPBIOS support
  *  22 - PNPBIOS support
  *  23 - APM BIOS support
- *  24 - APM BIOS support
+ *  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 ]
+ *  28 - stack_canary-20		[ for stack protector ]		<=== cacheline #8
  *  29 - unused
  *  30 - unused
  *  31 - TSS for double fault handler
  */
-#define GDT_ENTRY_TLS_MIN	6
-#define GDT_ENTRY_TLS_MAX 	(GDT_ENTRY_TLS_MIN + GDT_ENTRY_TLS_ENTRIES - 1)
+#define GDT_ENTRY_TLS_MIN		6
+#define GDT_ENTRY_TLS_MAX 		(GDT_ENTRY_TLS_MIN + GDT_ENTRY_TLS_ENTRIES - 1)
 
 #define GDT_ENTRY_KERNEL_CS		12
 #define GDT_ENTRY_KERNEL_DS		13
@@ -97,96 +106,134 @@
 
 #define GDT_ENTRY_DOUBLEFAULT_TSS	31
 
+/*
+ * Number of entries in the GDT table:
+ */
+#define GDT_ENTRIES			32
+
+/*
+ * Segment selector values corresponding to the above entries:
+ */
+
 #define __KERNEL_CS			(GDT_ENTRY_KERNEL_CS*8)
 #define __KERNEL_DS			(GDT_ENTRY_KERNEL_DS*8)
-#define __USER_DS			(GDT_ENTRY_DEFAULT_USER_DS*8+3)
-#define __USER_CS			(GDT_ENTRY_DEFAULT_USER_CS*8+3)
+#define __USER_DS			(GDT_ENTRY_DEFAULT_USER_DS*8 + 3)
+#define __USER_CS			(GDT_ENTRY_DEFAULT_USER_CS*8 + 3)
 #define __ESPFIX_SS			(GDT_ENTRY_ESPFIX_SS*8)
-#define PNP_CS32   (GDT_ENTRY_PNPBIOS_CS32 * 8) /* segment for calling fn */
-#define PNP_CS16   (GDT_ENTRY_PNPBIOS_CS16 * 8) /* code segment for BIOS */
+
+/* segment for calling fn: */
+#define PNP_CS32			(GDT_ENTRY_PNPBIOS_CS32*8)
+/* code segment for BIOS: */
+#define PNP_CS16			(GDT_ENTRY_PNPBIOS_CS16*8)
+
 /* "Is this PNP code selector (PNP_CS32 or PNP_CS16)?" */
-#define SEGMENT_IS_PNP_CODE(x)   (((x) & 0xf4) == PNP_CS32)
-#define PNP_DS     (GDT_ENTRY_PNPBIOS_DS * 8)	/* data segment for BIOS */
-#define PNP_TS1    (GDT_ENTRY_PNPBIOS_TS1 * 8)	/* transfer data segment */
-#define PNP_TS2    (GDT_ENTRY_PNPBIOS_TS2 * 8)	/* another data segment */
+#define SEGMENT_IS_PNP_CODE(x)		(((x) & 0xf4) == PNP_CS32)
+
+/* data segment for BIOS: */
+#define PNP_DS				(GDT_ENTRY_PNPBIOS_DS*8)
+/* transfer data segment: */
+#define PNP_TS1				(GDT_ENTRY_PNPBIOS_TS1*8)
+/* another data segment: */
+#define PNP_TS2				(GDT_ENTRY_PNPBIOS_TS2*8)
+
 #ifdef CONFIG_SMP
-#define __KERNEL_PERCPU			(GDT_ENTRY_PERCPU*8)
+# define __KERNEL_PERCPU		(GDT_ENTRY_PERCPU*8)
 #else
-#define __KERNEL_PERCPU			0
+# define __KERNEL_PERCPU		0
 #endif
+
 #ifdef CONFIG_CC_STACKPROTECTOR
-#define __KERNEL_STACK_CANARY		(GDT_ENTRY_STACK_CANARY*8)
+# define __KERNEL_STACK_CANARY		(GDT_ENTRY_STACK_CANARY*8)
 #else
-#define __KERNEL_STACK_CANARY		0
+# define __KERNEL_STACK_CANARY		0
 #endif
 
-#define GDT_ENTRIES 32
-
 #else /* 64-bit: */
 
 #include <asm/cache.h>
 
-#define GDT_ENTRY_KERNEL32_CS	1
-#define GDT_ENTRY_KERNEL_CS	2
-#define GDT_ENTRY_KERNEL_DS	3
+#define GDT_ENTRY_KERNEL32_CS		1
+#define GDT_ENTRY_KERNEL_CS		2
+#define GDT_ENTRY_KERNEL_DS		3
+
 /*
- * we cannot use the same code segment descriptor for user and kernel
- * -- not even in the long flat mode, because of different DPL /kkeil
- * GDT layout to get 64bit syscall/sysret right. sysret hardcodes selectors:
- * if returning to 32-bit userspace: cs = STAR.SYSRET_CS,
- * if returning to 64-bit userspace: cs = STAR.SYSRET_CS+16,
+ * We cannot use the same code segment descriptor for user and kernel mode,
+ * not even in long flat mode, because of different DPL.
+ *
+ * GDT layout to get 64-bit SYSCALL/SYSRET support right. SYSRET hardcodes
+ * selectors:
+ *
+ *   if returning to 32-bit userspace: cs = STAR.SYSRET_CS,
+ *   if returning to 64-bit userspace: cs = STAR.SYSRET_CS+16,
+ *
  * ss = STAR.SYSRET_CS+8 (in either case)
+ *
  * thus USER_DS should be between 32-bit and 64-bit code selectors:
  */
-#define GDT_ENTRY_DEFAULT_USER32_CS 4
-#define GDT_ENTRY_DEFAULT_USER_DS 5
-#define GDT_ENTRY_DEFAULT_USER_CS 6
+#define GDT_ENTRY_DEFAULT_USER32_CS	4
+#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_TSS		8  /* needs two entries */
-#define GDT_ENTRY_LDT		10 /* needs two entries */
-#define GDT_ENTRY_TLS_MIN	12
-#define GDT_ENTRY_TLS_MAX	14
+#define GDT_ENTRY_TLS_MIN		12
+#define GDT_ENTRY_TLS_MAX		14
 
-#define GDT_ENTRY_PER_CPU	15 /* abused to load per CPU data from limit */
+/* Abused to load per CPU data from limit */
+#define GDT_ENTRY_PER_CPU		15
 
-/* Selectors need to also have a correct RPL (+3 thingy) */
-#define __KERNEL_CS	(GDT_ENTRY_KERNEL_CS*8)
-#define __KERNEL_DS	(GDT_ENTRY_KERNEL_DS*8)
-#define __USER_DS	(GDT_ENTRY_DEFAULT_USER_DS*8+3)
-#define __USER_CS	(GDT_ENTRY_DEFAULT_USER_CS*8+3)
-#define __KERNEL32_CS	(GDT_ENTRY_KERNEL32_CS*8)
-#define __USER32_CS	(GDT_ENTRY_DEFAULT_USER32_CS*8+3)
-#define __USER32_DS	__USER_DS
-#define __PER_CPU_SEG	(GDT_ENTRY_PER_CPU*8+3)
+/*
+ * Number of entries in the GDT table:
+ */
+#define GDT_ENTRIES			16
 
-/* TLS indexes for 64bit - hardcoded in arch_prctl */
-#define FS_TLS 0
-#define GS_TLS 1
+/*
+ * Segment selector values corresponding to the above entries:
+ *
+ * Note, selectors also need to have a correct RPL,
+ * expressed with the +3 value for user-space selectors:
+ */
+#define __KERNEL32_CS			(GDT_ENTRY_KERNEL32_CS*8)
+#define __KERNEL_CS			(GDT_ENTRY_KERNEL_CS*8)
+#define __KERNEL_DS			(GDT_ENTRY_KERNEL_DS*8)
+#define __USER32_CS			(GDT_ENTRY_DEFAULT_USER32_CS*8 + 3)
+#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)
+#define __PER_CPU_SEG			(GDT_ENTRY_PER_CPU*8 + 3)
 
-#define GS_TLS_SEL ((GDT_ENTRY_TLS_MIN+GS_TLS)*8 + 3)
-#define FS_TLS_SEL ((GDT_ENTRY_TLS_MIN+FS_TLS)*8 + 3)
+/* TLS indexes for 64-bit - hardcoded in arch_prctl(): */
+#define FS_TLS				0
+#define GS_TLS				1
 
-#define GDT_ENTRIES 16
+#define GS_TLS_SEL			((GDT_ENTRY_TLS_MIN+GS_TLS)*8 + 3)
+#define FS_TLS_SEL			((GDT_ENTRY_TLS_MIN+FS_TLS)*8 + 3)
 
 #endif
 
 #ifndef CONFIG_PARAVIRT
-#define get_kernel_rpl()  0
+# define get_kernel_rpl()		0
 #endif
 
-#define IDT_ENTRIES 256
-#define NUM_EXCEPTION_VECTORS 32
-/* Bitmask of exception vectors which push an error code on the stack */
-#define EXCEPTION_ERRCODE_MASK  0x00027d00
-#define GDT_SIZE (GDT_ENTRIES * 8)
-#define GDT_ENTRY_TLS_ENTRIES 3
-#define TLS_SIZE (GDT_ENTRY_TLS_ENTRIES * 8)
+#define IDT_ENTRIES			256
+#define NUM_EXCEPTION_VECTORS		32
+
+/* Bitmask of exception vectors which push an error code on the stack: */
+#define EXCEPTION_ERRCODE_MASK		0x00027d00
+
+#define GDT_SIZE			(GDT_ENTRIES*8)
+#define GDT_ENTRY_TLS_ENTRIES		3
+#define TLS_SIZE			(GDT_ENTRY_TLS_ENTRIES* 8)
 
 #ifdef __KERNEL__
 #ifndef __ASSEMBLY__
+
 extern const char early_idt_handlers[NUM_EXCEPTION_VECTORS][2+2+5];
 #ifdef CONFIG_TRACING
-#define trace_early_idt_handlers early_idt_handlers
+# define trace_early_idt_handlers early_idt_handlers
 #endif
 
 /*
@@ -211,37 +258,30 @@ do {									\
 } while (0)
 
 /*
- * Save a segment register away
+ * Save a segment register away:
  */
 #define savesegment(seg, value)				\
 	asm("mov %%" #seg ",%0":"=r" (value) : : "memory")
 
 /*
- * x86_32 user gs accessors.
+ * x86-32 user GS accessors:
  */
 #ifdef CONFIG_X86_32
-#ifdef CONFIG_X86_32_LAZY_GS
-#define get_user_gs(regs)	(u16)({unsigned long v; savesegment(gs, v); v;})
-#define set_user_gs(regs, v)	loadsegment(gs, (unsigned long)(v))
-#define task_user_gs(tsk)	((tsk)->thread.gs)
-#define lazy_save_gs(v)		savesegment(gs, (v))
-#define lazy_load_gs(v)		loadsegment(gs, (v))
-#else	/* X86_32_LAZY_GS */
-#define get_user_gs(regs)	(u16)((regs)->gs)
-#define set_user_gs(regs, v)	do { (regs)->gs = (v); } while (0)
-#define task_user_gs(tsk)	(task_pt_regs(tsk)->gs)
-#define lazy_save_gs(v)		do { } while (0)
-#define lazy_load_gs(v)		do { } while (0)
-#endif	/* X86_32_LAZY_GS */
+# ifdef CONFIG_X86_32_LAZY_GS
+#  define get_user_gs(regs)		(u16)({ unsigned long v; savesegment(gs, v); v; })
+#  define set_user_gs(regs, v)		loadsegment(gs, (unsigned long)(v))
+#  define task_user_gs(tsk)		((tsk)->thread.gs)
+#  define lazy_save_gs(v)		savesegment(gs, (v))
+#  define lazy_load_gs(v)		loadsegment(gs, (v))
+# else	/* X86_32_LAZY_GS */
+#  define get_user_gs(regs)		(u16)((regs)->gs)
+#  define set_user_gs(regs, v)		do { (regs)->gs = (v); } while (0)
+#  define task_user_gs(tsk)		(task_pt_regs(tsk)->gs)
+#  define lazy_save_gs(v)		do { } while (0)
+#  define lazy_load_gs(v)		do { } while (0)
+# endif	/* X86_32_LAZY_GS */
 #endif	/* X86_32 */
 
-static inline unsigned long get_limit(unsigned long segment)
-{
-	unsigned long __limit;
-	asm("lsll %1,%0" : "=r" (__limit) : "r" (segment));
-	return __limit + 1;
-}
-
 #endif /* !__ASSEMBLY__ */
 #endif /* __KERNEL__ */