KVM: Fix race in apic->pending_events processing

apic->pending_events processing has a race that may cause INIT and
SIPI
processing to be reordered:

vpu0:                            vcpu1:
set INIT
                               test_and_clear_bit(KVM_APIC_INIT)
                                  process INIT
set INIT
set SIPI
                               test_and_clear_bit(KVM_APIC_SIPI)
                                  process SIPI

At the end INIT is left pending in pending_events. The following patch
fixes this by latching pending event before processing them.

Signed-off-by: Gleb Natapov <gleb@redhat.com>

1 files changed, 6 insertions, 3 deletions

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index e1adbb4aca75..0eee2c8b64d1 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1861,11 +1861,14 @@ void kvm_apic_accept_events(struct kvm_vcpu *vcpu)
 {
         struct kvm_lapic *apic = vcpu->arch.apic;
         unsigned int sipi_vector;
+        unsigned long pe;
 
-        if (!kvm_vcpu_has_lapic(vcpu))
+        if (!kvm_vcpu_has_lapic(vcpu) || !apic->pending_events)
                 return;
 
-        if (test_and_clear_bit(KVM_APIC_INIT, &apic->pending_events)) {
+        pe = xchg(&apic->pending_events, 0);
+
+        if (test_bit(KVM_APIC_INIT, &pe)) {
                 kvm_lapic_reset(vcpu);
                 kvm_vcpu_reset(vcpu);
                 if (kvm_vcpu_is_bsp(apic->vcpu))
@@ -1873,7 +1876,7 @@ void kvm_apic_accept_events(struct kvm_vcpu *vcpu)
                 else
                         vcpu->arch.mp_state = KVM_MP_STATE_INIT_RECEIVED;
         }
-        if (test_and_clear_bit(KVM_APIC_SIPI, &apic->pending_events) &&
+        if (test_bit(KVM_APIC_SIPI, &pe) &&
             vcpu->arch.mp_state == KVM_MP_STATE_INIT_RECEIVED) {
                 /* evaluate pending_events before reading the vector */
                 smp_rmb();