KVM: Fix last_guest_tsc / tsc_offset semantics
The variable last_guest_tsc was being used as an ad-hoc indicator that guest TSC has been initialized and recorded correctly. However, it may not have been, it could be that guest TSC has been set to some large value, the back to a small value (by, say, a software reboot). This defeats the logic and causes KVM to falsely assume that the guest TSC has gone backwards, marking the host TSC unstable, which is undesirable behavior. In addition, rather than try to compute an offset adjustment for the TSC on unstable platforms, just recompute the whole offset. This allows us to get rid of one callsite for adjust_tsc_offset, which is problematic because the units it takes are in guest units, but here, the computation was originally being done in host units. Doing this, and also recording last_guest_tsc when the TSC is written allow us to remove the tricky logic which depended on last_guest_tsc being zero to indicate a reset of uninitialized value. Instead, we now have the guarantee that the guest TSC offset is always at least something which will get us last_guest_tsc. Signed-off-by: Zachary Amsden <zamsden@gmail.com> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com> Signed-off-by: Avi Kivity <avi@redhat.com>
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@ -1079,6 +1079,7 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, u64 data)
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vcpu->arch.hv_clock.tsc_timestamp = 0;
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vcpu->arch.last_tsc_write = data;
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vcpu->arch.last_tsc_nsec = ns;
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vcpu->arch.last_guest_tsc = data;
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}
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EXPORT_SYMBOL_GPL(kvm_write_tsc);
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@ -1147,7 +1148,7 @@ static int kvm_guest_time_update(struct kvm_vcpu *v)
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* observed by the guest and ensure the new system time is greater.
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*/
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max_kernel_ns = 0;
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if (vcpu->hv_clock.tsc_timestamp && vcpu->last_guest_tsc) {
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if (vcpu->hv_clock.tsc_timestamp) {
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max_kernel_ns = vcpu->last_guest_tsc -
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vcpu->hv_clock.tsc_timestamp;
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max_kernel_ns = pvclock_scale_delta(max_kernel_ns,
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@ -2257,13 +2258,14 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
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u64 tsc;
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tsc = kvm_x86_ops->read_l1_tsc(vcpu);
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tsc_delta = !vcpu->arch.last_guest_tsc ? 0 :
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tsc - vcpu->arch.last_guest_tsc;
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tsc_delta = tsc - vcpu->arch.last_guest_tsc;
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if (tsc_delta < 0)
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mark_tsc_unstable("KVM discovered backwards TSC");
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if (check_tsc_unstable()) {
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kvm_x86_ops->adjust_tsc_offset(vcpu, -tsc_delta);
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u64 offset = kvm_x86_ops->compute_tsc_offset(vcpu,
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vcpu->arch.last_guest_tsc);
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kvm_x86_ops->write_tsc_offset(vcpu, offset);
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vcpu->arch.tsc_catchup = 1;
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}
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kvm_make_request(KVM_REQ_CLOCK_UPDATE, vcpu);
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