Convert the __this_cpu_ist_{top,bottom}_va() macros into proper functions,
and export __this_cpu_ist_top_va() to allow KVM to retrieve the top of the
per-CPU exception stack.

FRED introduced new fields in the host-state area of the VMCS for stack
levels 1->3 (HOST_IA32_FRED_RSP[123]), each respectively corresponding to
per-CPU exception stacks for #DB, NMI and #DF.  KVM must populate these
fields each time a vCPU is loaded onto a CPU.

To simplify access to the exception stacks in struct cea_exception_stacks,
a union is used to create an array alias, enabling array-style indexing of
the stack entries.

Signed-off-by: Xin Li (Intel) <xin@zytor.com>
---

Change in v7:
* Remove Suggested-bys (Dave Hansen).
* Move rename code in a separate patch (Dave Hansen).
* Access cea_exception_stacks using array indexing (Dave Hansen).
* Use BUILD_BUG_ON(ESTACK_DF != 0) to ensure the starting index is 0
  (Dave Hansen).

Change in v5:
* Export accessor instead of data (Christoph Hellwig).
* Add TB from Xuelian Guo.

Change in v4:
* Rewrite the change log and add comments to the export (Dave Hansen).
---
 arch/x86/include/asm/cpu_entry_area.h | 51 +++++++++++++--------------
 arch/x86/mm/cpu_entry_area.c          | 25 +++++++++++++
 2 files changed, 50 insertions(+), 26 deletions(-)

diff --git a/arch/x86/include/asm/cpu_entry_area.h b/arch/x86/include/asm/cpu_entry_area.h
index d0f884c28178..58cd71144e5e 100644
--- a/arch/x86/include/asm/cpu_entry_area.h
+++ b/arch/x86/include/asm/cpu_entry_area.h
@@ -16,6 +16,19 @@
 #define VC_EXCEPTION_STKSZ	0
 #endif
 
+/*
+ * The exception stack ordering in [cea_]exception_stacks
+ */
+enum exception_stack_ordering {
+	ESTACK_DF,
+	ESTACK_NMI,
+	ESTACK_DB,
+	ESTACK_MCE,
+	ESTACK_VC,
+	ESTACK_VC2,
+	N_EXCEPTION_STACKS
+};
+
 /* Macro to enforce the same ordering and stack sizes */
 #define ESTACKS_MEMBERS(guardsize, optional_stack_size)		\
 	char	ESTACK_DF_stack_guard[guardsize];		\
@@ -39,37 +52,29 @@ struct exception_stacks {
 
 /* The effective cpu entry area mapping with guard pages. */
 struct cea_exception_stacks {
-	ESTACKS_MEMBERS(PAGE_SIZE, EXCEPTION_STKSZ)
-};
-
-/*
- * The exception stack ordering in [cea_]exception_stacks
- */
-enum exception_stack_ordering {
-	ESTACK_DF,
-	ESTACK_NMI,
-	ESTACK_DB,
-	ESTACK_MCE,
-	ESTACK_VC,
-	ESTACK_VC2,
-	N_EXCEPTION_STACKS
+	union{
+		struct {
+			ESTACKS_MEMBERS(PAGE_SIZE, EXCEPTION_STKSZ)
+		};
+		struct {
+			char stack_guard[PAGE_SIZE];
+			char stack[EXCEPTION_STKSZ];
+		} event_stacks[N_EXCEPTION_STACKS];
+	};
 };
 
 #define CEA_ESTACK_SIZE(st)					\
 	sizeof(((struct cea_exception_stacks *)0)->st## _stack)
 
-#define CEA_ESTACK_BOT(ceastp, st)				\
-	((unsigned long)&(ceastp)->st## _stack)
-
-#define CEA_ESTACK_TOP(ceastp, st)				\
-	(CEA_ESTACK_BOT(ceastp, st) + CEA_ESTACK_SIZE(st))
-
 #define CEA_ESTACK_OFFS(st)					\
 	offsetof(struct cea_exception_stacks, st## _stack)
 
 #define CEA_ESTACK_PAGES					\
 	(sizeof(struct cea_exception_stacks) / PAGE_SIZE)
 
+extern unsigned long __this_cpu_ist_top_va(enum exception_stack_ordering stack);
+extern unsigned long __this_cpu_ist_bottom_va(enum exception_stack_ordering stack);
+
 #endif
 
 #ifdef CONFIG_X86_32
@@ -144,10 +149,4 @@ static __always_inline struct entry_stack *cpu_entry_stack(int cpu)
 	return &get_cpu_entry_area(cpu)->entry_stack_page.stack;
 }
 
-#define __this_cpu_ist_top_va(name)					\
-	CEA_ESTACK_TOP(__this_cpu_read(cea_exception_stacks), name)
-
-#define __this_cpu_ist_bottom_va(name)					\
-	CEA_ESTACK_BOT(__this_cpu_read(cea_exception_stacks), name)
-
 #endif
diff --git a/arch/x86/mm/cpu_entry_area.c b/arch/x86/mm/cpu_entry_area.c
index 9fa371af8abc..595c2e03ddd5 100644
--- a/arch/x86/mm/cpu_entry_area.c
+++ b/arch/x86/mm/cpu_entry_area.c
@@ -18,6 +18,31 @@ static DEFINE_PER_CPU_PAGE_ALIGNED(struct entry_stack_page, entry_stack_storage)
 static DEFINE_PER_CPU_PAGE_ALIGNED(struct exception_stacks, exception_stacks);
 DEFINE_PER_CPU(struct cea_exception_stacks*, cea_exception_stacks);
 
+/*
+ * FRED introduced new fields in the host-state area of the VMCS for
+ * stack levels 1->3 (HOST_IA32_FRED_RSP[123]), each respectively
+ * corresponding to per CPU stacks for #DB, NMI and #DF.  KVM must
+ * populate these each time a vCPU is loaded onto a CPU.
+ *
+ * Called from entry code, so must be noinstr.
+ */
+noinstr unsigned long __this_cpu_ist_bottom_va(enum exception_stack_ordering stack)
+{
+	struct cea_exception_stacks *s;
+
+	BUILD_BUG_ON(ESTACK_DF != 0);
+
+	s = __this_cpu_read(cea_exception_stacks);
+
+	return (unsigned long)&s->event_stacks[stack].stack;
+}
+
+noinstr unsigned long __this_cpu_ist_top_va(enum exception_stack_ordering stack)
+{
+	return __this_cpu_ist_bottom_va(stack) + EXCEPTION_STKSZ;
+}
+EXPORT_SYMBOL(__this_cpu_ist_top_va);
+
 static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, _cea_offset);
 
 static __always_inline unsigned int cea_offset(unsigned int cpu)
-- 
2.51.0