KVM: x86: Check kvm_rebooting in kvm_spurious_fault()

Explicitly check kvm_rebooting in kvm_spurious_fault() prior to invoking
BUG(), as opposed to assuming the caller has already done so.  Letting
kvm_spurious_fault() be called "directly" will allow VMX to better
optimize its low level assembly flows.

As a happy side effect, kvm_spurious_fault() no longer needs to be
marked as a dead end since it doesn't unconditionally BUG().
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1534,7 +1534,7 @@ enum {
 #define kvm_arch_vcpu_memslots_id(vcpu) ((vcpu)->arch.hflags & HF_SMM_MASK ? 1 : 0)
 #define kvm_memslots_for_spte_role(kvm, role) __kvm_memslots(kvm, (role).smm)
 
-asmlinkage void __noreturn kvm_spurious_fault(void);
+asmlinkage void kvm_spurious_fault(void);
 
 /*
  * Hardware virtualization extension instructions may fault if a

arch/x86/kvm/x86.c

@@ -360,6 +360,7 @@ EXPORT_SYMBOL_GPL(kvm_set_apic_base);
 asmlinkage __visible void kvm_spurious_fault(void)
 {
 	/* Fault while not rebooting.  We want the trace. */
+	if (!kvm_rebooting)
 		BUG();
 }
 EXPORT_SYMBOL_GPL(kvm_spurious_fault);

tools/objtool/check.c

@@ -138,7 +138,6 @@ static bool __dead_end_function(struct objtool_file *file, struct symbol *func,
 		"do_task_dead",
 		"__module_put_and_exit",
 		"complete_and_exit",
-		"kvm_spurious_fault",
 		"__reiserfs_panic",
 		"lbug_with_loc",
 		"fortify_panic",