KVM: x86: make vapics_in_nmi_mode atomic

commit 42720138 upstream.

Writes were a bit racy, but hard to turn into a bug at the same time.
(Particularly because modern Linux doesn't use this feature anymore.)
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
[Actually the next patch makes it much, much easier to trigger the race
 so I'm including this one for stable@ as well. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

arch/x86/include/asm/kvm_host.h

@@ -472,7 +472,7 @@ struct kvm_arch {
 	struct kvm_pic *vpic;
 	struct kvm_ioapic *vioapic;
 	struct kvm_pit *vpit;
-	int vapics_in_nmi_mode;
+	atomic_t vapics_in_nmi_mode;
 
 	unsigned int tss_addr;
 	struct page *apic_access_page;

arch/x86/kvm/i8254.c

@@ -318,7 +318,7 @@ static void pit_do_work(struct work_struct *work)
 		 * LVT0 to NMI delivery. Other PIC interrupts are just sent to
 		 * VCPU0, and only if its LVT0 is in EXTINT mode.
 		 */
-		if (kvm->arch.vapics_in_nmi_mode > 0)
+		if (atomic_read(&kvm->arch.vapics_in_nmi_mode) > 0)
 			kvm_for_each_vcpu(i, vcpu, kvm)
 				kvm_apic_nmi_wd_deliver(vcpu);
 	}

arch/x86/kvm/lapic.c

@@ -757,10 +757,10 @@ static void apic_manage_nmi_watchdog(struct kvm_lapic *apic, u32 lvt0_val)
 		if (!nmi_wd_enabled) {
 			apic_debug("Receive NMI setting on APIC_LVT0 "
 				   "for cpu %d\n", apic->vcpu->vcpu_id);
-			apic->vcpu->kvm->arch.vapics_in_nmi_mode++;
+			atomic_inc(&apic->vcpu->kvm->arch.vapics_in_nmi_mode);
 		}
 	} else if (nmi_wd_enabled)
-		apic->vcpu->kvm->arch.vapics_in_nmi_mode--;
+		atomic_dec(&apic->vcpu->kvm->arch.vapics_in_nmi_mode);
 }
 
 static int apic_reg_write(struct kvm_lapic *apic, u32 reg, u32 val)