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

Zachary Amsden
2009-12-15T21:25:07Z

On 12/15/2009 08:21 AM, Marcelo Tosatti wrote:
> 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).
>

Yes, that is an excellent point. The generation counter, the
tsc_synchronized variable and the per-vcpu clock counter all have some
amount of redundancy of information.

Perhaps instead of overlapping, they should be layered?

A rule for kvmclock: can't update kvmclock info until cpu is synchronized?

Thanks,

Zach