Re: [patch 2/2] KVM: fix kvm_vcpu_kick vs __vcpu_run race

[Marcelo Tosatti <mtosatti@redhat.com>]

On Fri, Apr 11, 2008 at 03:18:19PM +0300, Avi Kivity wrote:
> kvm_vcpu_kick() can be called from nonatomic contexts, so the vcpu->cpu 
> == cpu check is dangerous (and will warn on preemptible kernels, no?)

Doh, right. How's this.

-----------

KVM: fix kvm_vcpu_kick vs __vcpu_run race

There is a window open between testing of pending IRQ's 
and assignment of guest_mode in __vcpu_run.

Injection of IRQ's can race with __vcpu_run as follows:

CPU0                                CPU1
kvm_x86_ops->run()
vcpu->guest_mode = 0                SET_IRQ_LINE ioctl
..
kvm_x86_ops->inject_pending_irq     
kvm_cpu_has_interrupt()

                                    apic_test_and_set_irr()
                                    kvm_vcpu_kick
                                    if (vcpu->guest_mode)
                                        send_ipi()
                                    
vcpu->guest_mode = 1

So move guest_mode=1 assignment before ->inject_pending_irq, and make
sure that it won't reorder after it.


Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>


Index: kvm/arch/x86/kvm/x86.c
===================================================================
--- kvm.orig/arch/x86/kvm/x86.c
+++ kvm/arch/x86/kvm/x86.c
@@ -2777,6 +2777,13 @@ again:
 		goto out;
 	}
 
+	vcpu->guest_mode = 1;
+	/*
+	 * Make sure that guest_mode assignment won't happen after
+	 * testing the pending IRQ vector bitmap.
+	 */
+	smp_wmb();
+
 	if (vcpu->arch.exception.pending)
 		__queue_exception(vcpu);
 	else if (irqchip_in_kernel(vcpu->kvm))
@@ -2788,7 +2795,6 @@ again:
 
 	up_read(&vcpu->kvm->slots_lock);
 
-	vcpu->guest_mode = 1;
 	kvm_guest_enter();
 
 	if (vcpu->requests)
@@ -3944,11 +3950,13 @@ static void vcpu_kick_intr(void *info)
 void kvm_vcpu_kick(struct kvm_vcpu *vcpu)
 {
 	int ipi_pcpu = vcpu->cpu;
+	int cpu = get_cpu();
 
 	if (waitqueue_active(&vcpu->wq)) {
 		wake_up_interruptible(&vcpu->wq);
 		++vcpu->stat.halt_wakeup;
 	}
-	if (vcpu->guest_mode)
+	if (vcpu->guest_mode && vcpu->cpu != cpu)
 		smp_call_function_single(ipi_pcpu, vcpu_kick_intr, vcpu, 0, 0);
+	put_cpu();
 }

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