KVM: x86: hyper-v: Prevent using not-yet-updated TSC page by secondar…

…y CPUs

When KVM_REQ_MASTERCLOCK_UPDATE request is issued (e.g. after migration)
we need to make sure no vCPU sees stale values in PV clock structures and
thus all vCPUs are kicked with KVM_REQ_CLOCK_UPDATE. Hyper-V TSC page
clocksource is global and kvm_guest_time_update() only updates in on vCPU0
but this is not entirely correct: nothing blocks some other vCPU from
entering the guest before we finish the update on CPU0 and it can read
stale values from the page.

Invalidate TSC page in kvm_gen_update_masterclock() to switch all vCPUs
to using MSR based clocksource (HV_X64_MSR_TIME_REF_COUNT).

Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20210316143736.964151-3-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/hyperv.c

@@ -1137,6 +1137,29 @@ void kvm_hv_setup_tsc_page(struct kvm *kvm,
  mutex_unlock(&hv->hv_lock);
 }
 
+void kvm_hv_invalidate_tsc_page(struct kvm *kvm)
+{
+ struct kvm_hv *hv = to_kvm_hv(kvm);
+ u64 gfn;
+
+ if (!(hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE))
+ return;
+
+ mutex_lock(&hv->hv_lock);
+
+ if (!(hv->hv_tsc_page & HV_X64_MSR_TSC_REFERENCE_ENABLE))
+ goto out_unlock;
+
+ gfn = hv->hv_tsc_page >> HV_X64_MSR_TSC_REFERENCE_ADDRESS_SHIFT;
+
+ hv->tsc_ref.tsc_sequence = 0;
+ kvm_write_guest(kvm, gfn_to_gpa(gfn),
+ &hv->tsc_ref, sizeof(hv->tsc_ref.tsc_sequence));
+
+out_unlock:
+ mutex_unlock(&hv->hv_lock);
+}
+
 static int kvm_hv_set_msr_pw(struct kvm_vcpu *vcpu, u32 msr, u64 data,
  bool host)
 {

arch/x86/kvm/hyperv.h

@@ -133,6 +133,7 @@ void kvm_hv_process_stimers(struct kvm_vcpu *vcpu);
 
 void kvm_hv_setup_tsc_page(struct kvm *kvm,
  struct pvclock_vcpu_time_info *hv_clock);
+void kvm_hv_invalidate_tsc_page(struct kvm *kvm);
 
 void kvm_hv_init_vm(struct kvm *kvm);
 void kvm_hv_destroy_vm(struct kvm *kvm);

arch/x86/kvm/x86.c

@@ -2551,6 +2551,8 @@ static void kvm_gen_update_masterclock(struct kvm *kvm)
  struct kvm_vcpu *vcpu;
  struct kvm_arch *ka = &kvm->arch;
 
+ kvm_hv_invalidate_tsc_page(kvm);
+
  spin_lock(&ka->pvclock_gtod_sync_lock);
  kvm_make_mclock_inprogress_request(kvm);
  /* no guest entries from this point */