[PATCH v2] KVM: Fix simultaneous NMIs

Avi Kivity
2011-09-20T10:43:37Z

If simultaneous NMIs happen, we're supposed to queue the second
and next (collapsing them), but currently we sometimes collapse
the second into the first.

Fix by using a counter for pending NMIs instead of a bool; since
the counter limit depends on whether the processor is currently
in an NMI handler, which can only be checked in vcpu context
(via the NMI mask), we add a new KVM_REQ_NMI to request recalculation
of the counter.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 5 ++-
arch/x86/kvm/x86.c | 48 +++++++++++++++++++++++++-------------
include/linux/kvm_host.h | 1 +
3 files changed, 35 insertions(+), 19 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 6ab4241..ab62711 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -413,8 +413,9 @@ struct kvm_vcpu_arch {
u32 tsc_catchup_mult;
s8 tsc_catchup_shift;

- bool nmi_pending;
- bool nmi_injected;
+ atomic_t nmi_queued; /* unprocessed asynchronous NMIs */
+ unsigned nmi_pending; /* NMI queued after currently running handler */
+ bool nmi_injected; /* Trying to inject an NMI this entry */

struct mtrr_state_type mtrr_state;
u32 pat;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 6b37f18..d51e407 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -83,6 +83,7 @@
static void update_cr8_intercept(struct kvm_vcpu *vcpu);
static int kvm_dev_ioctl_get_supported_cpuid(struct kvm_cpuid2 *cpuid,
struct kvm_cpuid_entry2 __user *entries);
+static void process_nmi(struct kvm_vcpu *vcpu);

struct kvm_x86_ops *kvm_x86_ops;
EXPORT_SYMBOL_GPL(kvm_x86_ops);
@@ -359,8 +360,8 @@ void kvm_propagate_fault(struct kvm_vcpu *vcpu, struct x86_exception *fault)

void kvm_inject_nmi(struct kvm_vcpu *vcpu)
{
- kvm_make_request(KVM_REQ_EVENT, vcpu);
- vcpu->arch.nmi_pending = 1;
+ atomic_inc(&vcpu->arch.nmi_queued);
+ kvm_make_request(KVM_REQ_NMI, vcpu);
}
EXPORT_SYMBOL_GPL(kvm_inject_nmi);

@@ -2827,6 +2828,7 @@ static int kvm_vcpu_ioctl_x86_set_mce(struct kvm_vcpu *vcpu,
static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
struct kvm_vcpu_events *events)
{
+ process_nmi(vcpu);
events->exception.injected =
vcpu->arch.exception.pending &&
!kvm_exception_is_soft(vcpu->arch.exception.nr);
@@ -2844,7 +2846,7 @@ static void kvm_vcpu_ioctl_x86_get_vcpu_events(struct kvm_vcpu *vcpu,
KVM_X86_SHADOW_INT_MOV_SS | KVM_X86_SHADOW_INT_STI);

events->nmi.injected = vcpu->arch.nmi_injected;
- events->nmi.pending = vcpu->arch.nmi_pending;
+ events->nmi.pending = vcpu->arch.nmi_pending != 0;
events->nmi.masked = kvm_x86_ops->get_nmi_mask(vcpu);
events->nmi.pad = 0;

@@ -2864,6 +2866,7 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
| KVM_VCPUEVENT_VALID_SHADOW))
return -EINVAL;

+ process_nmi(vcpu);
vcpu->arch.exception.pending = events->exception.injected;
vcpu->arch.exception.nr = events->exception.nr;
vcpu->arch.exception.has_error_code = events->exception.has_error_code;
@@ -4763,7 +4766,7 @@ int kvm_inject_realmode_interrupt(struct kvm_vcpu *vcpu, int irq, int inc_eip)
kvm_set_rflags(vcpu, ctxt->eflags);

if (irq == NMI_VECTOR)
- vcpu->arch.nmi_pending = false;
+ vcpu->arch.nmi_pending = 0;
else
vcpu->arch.interrupt.pending = false;

@@ -5572,7 +5575,7 @@ static void inject_pending_event(struct kvm_vcpu *vcpu)
/* try to inject new event if pending */
if (vcpu->arch.nmi_pending) {
if (kvm_x86_ops->nmi_allowed(vcpu)) {
- vcpu->arch.nmi_pending = false;
+ --vcpu->arch.nmi_pending;
vcpu->arch.nmi_injected = true;
kvm_x86_ops->set_nmi(vcpu);
}
@@ -5604,10 +5607,26 @@ static void kvm_put_guest_xcr0(struct kvm_vcpu *vcpu)
}
}

+static void process_nmi(struct kvm_vcpu *vcpu)
+{
+ unsigned limit = 2;
+
+ /*
+ * x86 is limited to one NMI running, and one NMI pending after it.
+ * If an NMI is already in progress, limit further NMIs to just one.
+ * Otherwise, allow two (and we'll inject the first one immediately).
+ */
+ if (kvm_x86_ops->get_nmi_mask(vcpu) || vcpu->arch.nmi_injected)
+ limit = 1;
+
+ vcpu->arch.nmi_pending += atomic_xchg(&vcpu->arch.nmi_queued, 0);
+ vcpu->arch.nmi_pending = min(vcpu->arch.nmi_pending, limit);
+ kvm_make_request(KVM_REQ_EVENT, vcpu);
+}
+
static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
{
int r;
- bool nmi_pending;
bool req_int_win = !irqchip_in_kernel(vcpu->kvm) &&
vcpu->run->request_interrupt_window;

@@ -5647,6 +5666,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
}
if (kvm_check_request(KVM_REQ_STEAL_UPDATE, vcpu))
record_steal_time(vcpu);
+ if (kvm_check_request(KVM_REQ_NMI, vcpu))
+ process_nmi(vcpu);

}

@@ -5654,19 +5675,11 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
if (unlikely(r))
goto out;

- /*
- * An NMI can be injected between local nmi_pending read and
- * vcpu->arch.nmi_pending read inside inject_pending_event().
- * But in that case, KVM_REQ_EVENT will be set, which makes
- * the race described above benign.
- */
- nmi_pending = ACCESS_ONCE(vcpu->arch.nmi_pending);
-
if (kvm_check_request(KVM_REQ_EVENT, vcpu) || req_int_win) {
inject_pending_event(vcpu);

/* enable NMI/IRQ window open exits if needed */
- if (nmi_pending)
+ if (vcpu->arch.nmi_pending)
kvm_x86_ops->enable_nmi_window(vcpu);
else if (kvm_cpu_has_interrupt(vcpu) || req_int_win)
kvm_x86_ops->enable_irq_window(vcpu);
@@ -6374,7 +6387,8 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)

int kvm_arch_vcpu_reset(struct kvm_vcpu *vcpu)
{
- vcpu->arch.nmi_pending = false;
+ atomic_set(&vcpu->arch.nmi_queued, 0);
+ vcpu->arch.nmi_pending = 0;
vcpu->arch.nmi_injected = false;

vcpu->arch.switch_db_regs = 0;
@@ -6649,7 +6663,7 @@ int kvm_arch_vcpu_runnable(struct kvm_vcpu *vcpu)
!vcpu->arch.apf.halted)
|| !list_empty_careful(&vcpu->async_pf.done)
|| vcpu->arch.mp_state == KVM_MP_STATE_SIPI_RECEIVED
- || vcpu->arch.nmi_pending ||
+ || atomic_read(&vcpu->arch.nmi_queued) ||
(kvm_arch_interrupt_allowed(vcpu) &&
kvm_cpu_has_interrupt(vcpu));
}
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 2a414f6..d526231 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -49,6 +49,7 @@
#define KVM_REQ_EVENT 11
#define KVM_REQ_APF_HALT 12
#define KVM_REQ_STEAL_UPDATE 13
+#define KVM_REQ_NMI 14

#define KVM_USERSPACE_IRQ_SOURCE_ID 0

--
1.7.6.3