x86/gdt: Use bitfields for initialization

The GDT entry related code uses two ways to access entries via
union fields:

 - bitfields

 - macros which initialize the two 16-bit parts of the entry
   by magic shift and mask operations.

Clean it up and only use the bitfields to initialize and access entries.

( The old access patterns were partly done due to GCC optimizing bitfield
  accesses in a horrible way - that's mostly fixed these days and clarity
  of code in such low level accessors is very important. )

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20170828064958.197673367@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/entry/vdso/vma.c

@@ -351,7 +351,7 @@ static void vgetcpu_cpu_init(void *arg)
 	 * and 8 bits for the node)
 	 */
 	d.limit0 = cpu | ((node & 0xf) << 12);
-	d.limit = node >> 4;
+	d.limit1 = node >> 4;
 	d.type = 5;		/* RO data, expand down, accessed */
 	d.dpl = 3;		/* Visible to user code */
 	d.s = 1;		/* Not a system segment */

arch/x86/include/asm/desc.h

@@ -23,7 +23,7 @@ static inline void fill_ldt(struct desc_struct *desc, const struct user_desc *in
 	desc->s			= 1;
 	desc->dpl		= 0x3;
 	desc->p			= info->seg_not_present ^ 1;
-	desc->limit		= (info->limit & 0xf0000) >> 16;
+	desc->limit1		= (info->limit & 0xf0000) >> 16;
 	desc->avl		= info->useable;
 	desc->d			= info->seg_32bit;
 	desc->g			= info->limit_in_pages;
@@ -170,14 +170,20 @@ static inline void pack_descriptor(struct desc_struct *desc, unsigned long base,
 				   unsigned long limit, unsigned char type,
 				   unsigned char flags)
 {
-	desc->a = ((base & 0xffff) << 16) | (limit & 0xffff);
-	desc->b = (base & 0xff000000) | ((base & 0xff0000) >> 16) |
-		(limit & 0x000f0000) | ((type & 0xff) << 8) |
-		((flags & 0xf) << 20);
-	desc->p = 1;
+	desc->limit0		= (u16) limit;
+	desc->base0		= (u16) base;
+	desc->base1		= (base >> 16) & 0xFF;
+	desc->type		= type & 0x0F;
+	desc->s			= 0;
+	desc->dpl		= 0;
+	desc->p			= 1;
+	desc->limit1		= (limit >> 16) & 0xF;
+	desc->avl		= (flags >> 0) & 0x01;
+	desc->l			= (flags >> 1) & 0x01;
+	desc->d			= (flags >> 2) & 0x01;
+	desc->g			= (flags >> 3) & 0x01;
 }
 
-
 static inline void set_tssldt_descriptor(void *d, unsigned long addr,
 					 unsigned type, unsigned size)
 {
@@ -195,7 +201,7 @@ static inline void set_tssldt_descriptor(void *d, unsigned long addr,
 	desc->base2		= (addr >> 24) & 0xFF;
 	desc->base3		= (u32) (addr >> 32);
 #else
-	pack_descriptor((struct desc_struct *)d, addr, size, 0x80 | type, 0);
+	pack_descriptor((struct desc_struct *)d, addr, size, type, 0);
 #endif
 }
 
@@ -395,13 +401,13 @@ static inline void set_desc_base(struct desc_struct *desc, unsigned long base)
 
 static inline unsigned long get_desc_limit(const struct desc_struct *desc)
 {
-	return desc->limit0 | (desc->limit << 16);
+	return desc->limit0 | (desc->limit1 << 16);
 }
 
 static inline void set_desc_limit(struct desc_struct *desc, unsigned long limit)
 {
 	desc->limit0 = limit & 0xffff;
-	desc->limit = (limit >> 16) & 0xf;
+	desc->limit1 = (limit >> 16) & 0xf;
 }
 
 #ifdef CONFIG_X86_64

arch/x86/include/asm/desc_defs.h

@@ -11,34 +11,30 @@
 
 #include <linux/types.h>
 
-/*
- * FIXME: Accessing the desc_struct through its fields is more elegant,
- * and should be the one valid thing to do. However, a lot of open code
- * still touches the a and b accessors, and doing this allow us to do it
- * incrementally. We keep the signature as a struct, rather than a union,
- * so we can get rid of it transparently in the future -- glommer
- */
 /* 8 byte segment descriptor */
 struct desc_struct {
-	union {
-		struct {
-			unsigned int a;
-			unsigned int b;
-		};
-		struct {
-			u16 limit0;
-			u16 base0;
-			unsigned base1: 8, type: 4, s: 1, dpl: 2, p: 1;
-			unsigned limit: 4, avl: 1, l: 1, d: 1, g: 1, base2: 8;
-		};
-	};
+	u16	limit0;
+	u16	base0;
+	u16	base1: 8, type: 4, s: 1, dpl: 2, p: 1;
+	u16	limit1: 4, avl: 1, l: 1, d: 1, g: 1, base2: 8;
 } __attribute__((packed));
 
-#define GDT_ENTRY_INIT(flags, base, limit) { { { \
-		.a = ((limit) & 0xffff) | (((base) & 0xffff) << 16), \
-		.b = (((base) & 0xff0000) >> 16) | (((flags) & 0xf0ff) << 8) | \
-			((limit) & 0xf0000) | ((base) & 0xff000000), \
-	} } }
+#define GDT_ENTRY_INIT(flags, base, limit)			\
+	{							\
+		.limit0		= (u16) (limit),		\
+		.limit1		= ((limit) >> 16) & 0x0F,	\
+		.base0		= (u16) (base),			\
+		.base1		= ((base) >> 16) & 0xFF,	\
+		.base2		= ((base) >> 24) & 0xFF,	\
+		.type		= (flags & 0x0f),		\
+		.s		= (flags >> 4) & 0x01,		\
+		.dpl		= (flags >> 5) & 0x03,		\
+		.p		= (flags >> 7) & 0x01,		\
+		.avl		= (flags >> 12) & 0x01,		\
+		.l		= (flags >> 13) & 0x01,		\
+		.d		= (flags >> 14) & 0x01,		\
+		.g		= (flags >> 15) & 0x01,		\
+	}
 
 enum {
 	GATE_INTERRUPT = 0xE,

arch/x86/math-emu/fpu_system.h

@@ -49,7 +49,7 @@ static inline unsigned long seg_get_base(struct desc_struct *d)
 
 static inline unsigned long seg_get_limit(struct desc_struct *d)
 {
-	return ((unsigned long)d->limit << 16) | d->limit0;
+	return ((unsigned long)d->limit1 << 16) | d->limit0;
 }
 
 static inline unsigned long seg_get_granularity(struct desc_struct *d)