KVM: protect assigned dev workqueue, int handler and irq acker

Commit Message

Marcelo Tosatti Feb. 26, 2009, 11:50 p.m. UTC

Can someone with HW test this please?

-----

kvm_assigned_dev_ack_irq is vulnerable to a race condition with the
interrupt handler function. It does:

        if (dev->host_irq_disabled) {
                enable_irq(dev->host_irq);
                dev->host_irq_disabled = false;
	}

If an interrupt triggers before the host->dev_irq_disabled assignment,
it will disable the interrupt and set dev->host_irq_disabled to true.

On return to kvm_assigned_dev_ack_irq, dev->host_irq_disabled is set to
false, and the next kvm_assigned_dev_ack_irq call will fail to reenable
it.

Other than that, having the interrupt handler and work handlers run in
parallel sounds like asking for trouble (could not spot any obvious
problem, but better not have to, its fragile).



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Comments

Yang, Sheng Feb. 27, 2009, 4:17 a.m. UTC | #1

On Friday 27 February 2009 07:50:54 Marcelo Tosatti wrote:
> Can someone with HW test this please?

Good catch! The patch works fine on my side.

Can it be a per-device lock? One big lock for all assigned device seems 
restrict scalability. 

> -----
>
> kvm_assigned_dev_ack_irq is vulnerable to a race condition with the
> interrupt handler function. It does:
>
>         if (dev->host_irq_disabled) {
>                 enable_irq(dev->host_irq);
>                 dev->host_irq_disabled = false;
> 	}
>
> If an interrupt triggers before the host->dev_irq_disabled assignment,
> it will disable the interrupt and set dev->host_irq_disabled to true.
>
> On return to kvm_assigned_dev_ack_irq, dev->host_irq_disabled is set to
> false, and the next kvm_assigned_dev_ack_irq call will fail to reenable
> it.
>
> Other than that, having the interrupt handler and work handlers run in
> parallel sounds like asking for trouble (could not spot any obvious
> problem, but better not have to, its fragile).

Well, my original purpose is a FIFO between interrupt handler and work(for 
MSI-X), but seems too complex... And I also don't see any problem for now...

Patch

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 3832243..faaf386 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -152,6 +152,7 @@  struct kvm {
 	unsigned long mmu_notifier_seq;
 	long mmu_notifier_count;
 #endif
+	spinlock_t assigned_dev_lock;
 };
 
 /* The guest did something we don't support. */
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 4d2be16..2bbf074 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -41,6 +41,7 @@ 
 #include <linux/pagemap.h>
 #include <linux/mman.h>
 #include <linux/swap.h>
+#include <linux/spinlock.h>
 
 #include <asm/processor.h>
 #include <asm/io.h>
@@ -132,6 +133,7 @@  static void kvm_assigned_dev_interrupt_work_handler(struct work_struct *work)
 	 * finer-grained lock, update this
 	 */
 	mutex_lock(&kvm->lock);
+	spin_lock_irq(&kvm->assigned_dev_lock);
 	if (assigned_dev->irq_requested_type & KVM_ASSIGNED_DEV_MSIX) {
 		struct kvm_guest_msix_entry *guest_entries =
 			assigned_dev->guest_msix_entries;
@@ -158,18 +160,21 @@  static void kvm_assigned_dev_interrupt_work_handler(struct work_struct *work)
 		}
 	}
 
+	spin_unlock_irq(&kvm->assigned_dev_lock);
 	mutex_unlock(&assigned_dev->kvm->lock);
 }
 
 static irqreturn_t kvm_assigned_dev_intr(int irq, void *dev_id)
 {
+	unsigned long flags;
 	struct kvm_assigned_dev_kernel *assigned_dev =
 		(struct kvm_assigned_dev_kernel *) dev_id;
 
+	spin_lock_irqsave(&assigned_dev->kvm->assigned_dev_lock, flags);
 	if (assigned_dev->irq_requested_type == KVM_ASSIGNED_DEV_MSIX) {
 		int index = find_index_from_host_irq(assigned_dev, irq);
 		if (index < 0)
-			return IRQ_HANDLED;
+			goto out;
 		assigned_dev->guest_msix_entries[index].flags |=
 			KVM_ASSIGNED_MSIX_PENDING;
 	}
@@ -179,6 +184,8 @@  static irqreturn_t kvm_assigned_dev_intr(int irq, void *dev_id)
 	disable_irq_nosync(irq);
 	assigned_dev->host_irq_disabled = true;
 
+out:
+	spin_unlock_irqrestore(&assigned_dev->kvm->assigned_dev_lock, flags);
 	return IRQ_HANDLED;
 }
 
@@ -186,6 +193,7 @@  static irqreturn_t kvm_assigned_dev_intr(int irq, void *dev_id)
 static void kvm_assigned_dev_ack_irq(struct kvm_irq_ack_notifier *kian)
 {
 	struct kvm_assigned_dev_kernel *dev;
+	unsigned long flags;
 
 	if (kian->gsi == -1)
 		return;
@@ -198,10 +206,12 @@  static void kvm_assigned_dev_ack_irq(struct kvm_irq_ack_notifier *kian)
 	/* The guest irq may be shared so this ack may be
 	 * from another device.
 	 */
+	spin_lock_irqsave(&dev->kvm->assigned_dev_lock, flags);
 	if (dev->host_irq_disabled) {
 		enable_irq(dev->host_irq);
 		dev->host_irq_disabled = false;
 	}
+	spin_unlock_irqrestore(&dev->kvm->assigned_dev_lock, flags);
 }
 
 /* The function implicit hold kvm->lock mutex due to cancel_work_sync() */
@@ -955,6 +965,7 @@  static struct kvm *kvm_create_vm(void)
 	kvm->mm = current->mm;
 	atomic_inc(&kvm->mm->mm_count);
 	spin_lock_init(&kvm->mmu_lock);
+	spin_lock_init(&kvm->assigned_dev_lock);
 	kvm_io_bus_init(&kvm->pio_bus);
 	mutex_init(&kvm->lock);
 	kvm_io_bus_init(&kvm->mmio_bus);