KVM: x86: move steal time initialization to vcpu entry time

As reported at https://bugs.launchpad.net/qemu/+bug/1494350,
it is possible to have vcpu->arch.st.last_steal initialized from a thread other than vcpu thread, say the iothread, via KVM_SET_MSRS.

Which can cause an overflow later (when subtracting from vcpu threads sched_info.run_delay).

To avoid that, move steal time accumulation to vcpu entry time, before copying steal time data to guest.

Signed-off-by Marcelo Tosatti <[email protected]>

arch/x86/kvm/x86.c +2 -8

index 8f0f6ec..0e0332e 100644
@@ -2030,6 +2030,8 @@ static void accumulate_steal_time(struct kvm_vcpu *vcpu)
 
 static void record_steal_time(struct kvm_vcpu *vcpu)
 {
+       accumulate_steal_time(vcpu);
+
        if (!(vcpu->arch.st.msr_val & KVM_MSR_ENABLED))
                return;
 

@@ -2182,12 +2184,6 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
                if (!(data & KVM_MSR_ENABLED))
                        break;
 
-               vcpu->arch.st.last_steal = current->sched_info.run_delay;
-
-               preempt_disable();
-               accumulate_steal_time(vcpu);
-               preempt_enable();
-
                kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
 
                break;

@@ -2830,7 +2826,6 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
                vcpu->cpu = cpu;
        }
 
-       accumulate_steal_time(vcpu);
        kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
 }
 
--

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Quoted 8 lines

As reported at https://bugs.launchpad.net/qemu/+bug/1494350,
it is possible to have vcpu->arch.st.last_steal initialized from a thread other than vcpu thread, say the iothread, via KVM_SET_MSRS.

Which can cause an overflow later (when subtracting from vcpu threads sched_info.run_delay).

Quoted 4 lines

To avoid that, move steal time accumulation to vcpu entry time, before copying steal time data to guest.

Signed-off-by Marcelo Tosatti <[email protected]>

Reviewed-by David Matlack <[email protected]>