[PATCH v8 05/21] x86/cea: Export API for per-CPU exception stacks for KVM

Xin Li (Intel)
2025-10-14T01:11:01Z

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