KVM: nVMX: Open a window for pending nested VMX preemption timer

Add a kvm_x86_ops hook to detect a nested pending "hypervisor timer" and
use it to effectively open a window for servicing the expired timer.
Like pending SMIs on VMX, opening a window simply means requesting an
immediate exit.

This fixes a bug where an expired VMX preemption timer (for L2) will be
delayed and/or lost if a pending exception is injected into L2.  The
pending exception is rightly prioritized by vmx_check_nested_events()
and injected into L2, with the preemption timer left pending.  Because
no window opened, L2 is free to run uninterrupted.

Fixes: f4124500 ("KVM: nVMX: Fully emulate preemption timer")
Reported-by: Jim Mattson <jmattson@google.com>
Cc: Oliver Upton <oupton@google.com>
Cc: Peter Shier <pshier@google.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Message-Id: <20200423022550.15113-3-sean.j.christopherson@intel.com>
[Check it in kvm_vcpu_has_events too, to ensure that the preemption
 timer is serviced promptly even if the vCPU is halted and L1 is not
 intercepting HLT. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -1256,6 +1256,7 @@ struct kvm_x86_ops {
 
 struct kvm_x86_nested_ops {
 	int (*check_events)(struct kvm_vcpu *vcpu);
+	bool (*hv_timer_pending)(struct kvm_vcpu *vcpu);
 	int (*get_state)(struct kvm_vcpu *vcpu,
 			 struct kvm_nested_state __user *user_kvm_nested_state,
 			 unsigned user_data_size);

arch/x86/kvm/vmx/nested.c

@@ -3687,6 +3687,12 @@ static void nested_vmx_update_pending_dbg(struct kvm_vcpu *vcpu)
 			    vcpu->arch.exception.payload);
 }
 
+static bool nested_vmx_preemption_timer_pending(struct kvm_vcpu *vcpu)
+{
+	return nested_cpu_has_preemption_timer(get_vmcs12(vcpu)) &&
+	       to_vmx(vcpu)->nested.preemption_timer_expired;
+}
+
 static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
@@ -3742,8 +3748,7 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
 		return 0;
 	}
 
-	if (nested_cpu_has_preemption_timer(get_vmcs12(vcpu)) &&
-	    vmx->nested.preemption_timer_expired) {
+	if (nested_vmx_preemption_timer_pending(vcpu)) {
 		if (block_nested_events)
 			return -EBUSY;
 		nested_vmx_vmexit(vcpu, EXIT_REASON_PREEMPTION_TIMER, 0, 0);
@@ -6448,6 +6453,7 @@ __init int nested_vmx_hardware_setup(struct kvm_x86_ops *ops,
 
 struct kvm_x86_nested_ops vmx_nested_ops = {
 	.check_events = vmx_check_nested_events,
+	.hv_timer_pending = nested_vmx_preemption_timer_pending,
 	.get_state = vmx_get_nested_state,
 	.set_state = vmx_set_nested_state,
 	.get_vmcs12_pages = nested_get_vmcs12_pages,

arch/x86/kvm/x86.c

@@ -8355,6 +8355,10 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 				kvm_x86_ops.enable_nmi_window(vcpu);
 			if (kvm_cpu_has_injectable_intr(vcpu) || req_int_win)
 				kvm_x86_ops.enable_irq_window(vcpu);
+			if (is_guest_mode(vcpu) &&
+			    kvm_x86_ops.nested_ops->hv_timer_pending &&
+			    kvm_x86_ops.nested_ops->hv_timer_pending(vcpu))
+				req_immediate_exit = true;
 			WARN_ON(vcpu->arch.exception.pending);
 		}
 
@@ -10211,6 +10215,11 @@ static inline bool kvm_vcpu_has_events(struct kvm_vcpu *vcpu)
 	if (kvm_hv_has_stimer_pending(vcpu))
 		return true;
 
+	if (is_guest_mode(vcpu) &&
+	    kvm_x86_ops.nested_ops->hv_timer_pending &&
+	    kvm_x86_ops.nested_ops->hv_timer_pending(vcpu))
+		return true;
+
 	return false;
 }