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

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Date:	Tue, 15 Dec 2009 11:26:59 -1000
From:	Zachary Amsden <zamsden@...hat.com>
To:	Marcelo Tosatti <mtosatti@...hat.com>
CC:	kvm@...r.kernel.org, Avi Kivity <avi@...hat.com>,
	Joerg Roedel <joerg.roedel@....com>,
	linux-kernel@...r.kernel.org, Dor Laor <dlaor@...hat.com>
Subject: Re: [PATCH RFC: kvm tsc virtualization 15/20] Fix longstanding	races

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
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