Re: [PATCH RFC: kvm tsc virtualization 15/20] Fix longstanding races

Marcelo Tosatti
2009-12-15T18:23:19Z

On Mon, Dec 14, 2009 at 06:08:42PM -1000, Zachary Amsden wrote:

<snip>

+ atomic_set(&per_cpu(cpu_tsc_synchronized, freq->cpu),
0);
+ spin_lock(&kvm_lock);
+ list_for_each_entry(kvm, &vm_list, vm_list) {
+ kvm_for_each_vcpu(i, vcpu, kvm) {
+ if (vcpu->cpu != freq->cpu)
+ continue;
+ if (vcpu->cpu != smp_processor_id())
+ send_ipi++;
+ kvm_request_guest_time_update(vcpu);

There is some overlap here between KVM_REQ_KVMCLOCK_UPDATE and
cpu_tsc_synchronized. Its the same information (frequency for a CPU has
changed) stored in two places.

Later you do:

spin_lock(&kvm_lock);
list_for_each_entry(kvm, &vm_list, vm_list) {
kvm_for_each_vcpu(i, vcpu, kvm) {
if (vcpu->cpu != freq->cpu)
continue;
if (vcpu->cpu != smp_processor_id())
send_ipi++;
kvm_request_guest_time_update(vcpu);
}
}
spin_unlock(&kvm_lock);
<--- a remote CPU could have updated kvmclock information
with stale cpu_tsc_khz, clearing the
KVM_REQ_KVMCLOCK_UPDATE bit.
smp_call_function(evict) (which sets cpu_tsc_synchronized
to zero)

Maybe worthwhile to unify it. Perhaps use the per cpu tsc generation in
addition to vcpu_load to update kvmclock info (on arch vcpu_load update
kvmclock store generation, update again on generation change).