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

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Date:	Tue, 15 Dec 2009 16:21:39 -0200
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Zachary Amsden <zamsden@...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 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).

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