Greetings,
I noticed a race condition when running two guests simultaneously and
debugging both guests (on 64-bit intel cpus). Periodically I would get
errors from the vmread, vmwrite, or vmresume instructions. Some research
revealed that these errors were being caused by having an invalid vmcs
loaded. Further, I found that the vmcs is a per_cpu variable, which I
believe means that any reference to it is invalid after a context
switch. (Corrections appreciated). This means that the vmcs must be
reloaded each time the process is switched to. The patch below fixed the
problem for me.
This patch does three things.
1. Extends the critical section in __vcpu_run to include the handling of
vmexits, where many of the vmread/writes occur.
2. Perform a vcpu_load after we enter the critical section, and after we
return from kvm_resched.
3. Move the call to kvm_guest_debug_pre into the critical section
(because it calls vmread/write).
I hope you find this useful. I am not on list, so please CC me on replies.
~Jesse Dutton
diff -ruNa kvm-72/kernel/x86.c kvm-72-changed/kernel/x86.c
--- kvm-72/kernel/x86.c 2008-07-27 06:20:10.000000000 -0700
+++ kvm-72-changed/kernel/x86.c 2008-07-31 15:25:25.000000000 -0700
@@ -2845,8 +2845,6 @@
vapic_enter(vcpu);
preempted:
- if (vcpu->guest_debug.enabled)
- kvm_x86_ops->guest_debug_pre(vcpu);
again:
if (vcpu->requests)
@@ -2878,7 +2876,12 @@
clear_bit(KVM_REQ_PENDING_TIMER, &vcpu->requests);
kvm_inject_pending_timer_irqs(vcpu);
+ vcpu_put(vcpu);
preempt_disable();
+ vcpu_load(vcpu);
+
+ if (vcpu->guest_debug.enabled)
+ kvm_x86_ops->guest_debug_pre(vcpu);
kvm_x86_ops->prepare_guest_switch(vcpu);
kvm_load_guest_fpu(vcpu);
@@ -2941,7 +2944,6 @@
kvm_guest_exit();
- preempt_enable();
down_read(&vcpu->kvm->slots_lock);
@@ -2960,6 +2962,8 @@
r = kvm_x86_ops->handle_exit(kvm_run, vcpu);
+ preempt_enable();
+
if (r > 0) {
if (dm_request_for_irq_injection(vcpu, kvm_run)) {
r = -EINTR;
@@ -2974,7 +2978,9 @@
out:
up_read(&vcpu->kvm->slots_lock);
if (r > 0) {
+ vcpu_put(vcpu);
kvm_resched(vcpu);
+ vcpu_load(vcpu);
down_read(&vcpu->kvm->slots_lock);
goto preempted;
}
--
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