KVM: x86: make vapics_in_nmi_mode atomic

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]
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

3 files changed, 4 insertions, 4 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c7fa57b529d2..2a7f5d782c33 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -607,7 +607,7 @@ 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;
         struct mutex apic_map_lock;
         struct kvm_apic_map *apic_map;
 
diff --git a/arch/x86/kvm/i8254.c b/arch/x86/kvm/i8254.c
index 4dce6f8b6129..f90952f64e79 100644
--- a/arch/x86/kvm/i8254.c
+++ b/arch/x86/kvm/i8254.c
@@ -305,7 +305,7 @@ static void pit_do_work(struct kthread_work *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);
         }
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 36e9de1b4127..607a56b35327 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1263,10 +1263,10 @@ 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)