[PATCH 2/4] x86/entry: Use asm_noist_exc_nmi() for NMI in early booting stage

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

While the other entries for the exceptions which use Interrupt stacks can
be also used on the kernel stack, asm_exc_nmi() can not be used on the
kernel stack for it relies on the RSP-located "NMI executing" variable
which expects to on a fixed location in the NMI IST stack.  When it is
unexpectedly called from the kernel stack, the RSP-located "NMI executing"
variable is also on the kernel stack and is "uninitialized" and can cause
the NMI entry to run in the wrong way.

Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Uros Bizjak <ubizjak@gmail.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/include/asm/idtentry.h | 2 --
 arch/x86/kernel/idt.c           | 8 +++++++-
 arch/x86/kernel/nmi.c           | 7 ++++---
 3 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/idtentry.h b/arch/x86/include/asm/idtentry.h
index 5b11d2ddbb5c..0831c0da5957 100644
--- a/arch/x86/include/asm/idtentry.h
+++ b/arch/x86/include/asm/idtentry.h
@@ -589,9 +589,7 @@ DECLARE_IDTENTRY_RAW(X86_TRAP_MC,	xenpv_exc_machine_check);
 
 /* NMI */
 DECLARE_IDTENTRY_NMI(X86_TRAP_NMI,	exc_nmi);
-#ifdef CONFIG_XEN_PV
 DECLARE_IDTENTRY_RAW(X86_TRAP_NMI,	noist_exc_nmi);
-#endif
 
 /* #DB */
 #ifdef CONFIG_X86_64
diff --git a/arch/x86/kernel/idt.c b/arch/x86/kernel/idt.c
index d552f177eca0..c75409633f16 100644
--- a/arch/x86/kernel/idt.c
+++ b/arch/x86/kernel/idt.c
@@ -71,10 +71,16 @@ static const __initconst struct idt_data early_idts[] = {
  * cpu_init() is invoked. Interrupt stacks cannot be used at that point and
  * the traps which use them are reinitialized with IST after cpu_init() has
  * set up TSS.
+ *
+ * While the other entries for the exceptions which use Interrupt stacks can
+ * be also used on the kernel stack, asm_exc_nmi() can not be used on the
+ * kernel stack for it relies on the RSP-located "NMI executing" variable
+ * which expects to on a fixed location in the NMI IST stack.  For early
+ * booting stage, asm_noist_exc_nmi() is used for NMI.
  */
 static const __initconst struct idt_data def_idts[] = {
 	INTG(X86_TRAP_DE,		asm_exc_divide_error),
-	INTG(X86_TRAP_NMI,		asm_exc_nmi),
+	INTG(X86_TRAP_NMI,		asm_noist_exc_nmi),
 	INTG(X86_TRAP_BR,		asm_exc_bounds),
 	INTG(X86_TRAP_UD,		asm_exc_invalid_op),
 	INTG(X86_TRAP_NM,		asm_exc_device_not_available),
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index 2b907a76d0a1..2fb1fd59d714 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -524,13 +524,14 @@ DEFINE_IDTENTRY_RAW(exc_nmi)
 		mds_user_clear_cpu_buffers();
 }
 
-#ifdef CONFIG_XEN_PV
 DEFINE_IDTENTRY_RAW(noist_exc_nmi)
 {
-	/* On Xen PV, NMI doesn't use IST.  The C part is the same as native. */
+	/*
+	 * On Xen PV and early booting stage, NMI doesn't use IST.
+	 * The C part is the same as native.
+	 */
 	exc_nmi(regs);
 }
-#endif
 
 void stop_nmi(void)
 {
-- 
2.19.1.6.gb485710b