KVM: nVMX: Request immediate exit iff pending nested event needs injection

When requesting an immediate exit from L2 in order to inject a pending
event, do so only if the pending event actually requires manual injection,
i.e. if and only if KVM actually needs to regain control in order to
deliver the event.

Avoiding the "immediate exit" isn't simply an optimization, it's necessary
to make forward progress, as the "already expired" VMX preemption timer
trick that KVM uses to force a VM-Exit has higher priority than events
that aren't directly injected.

At present time, this is a glorified nop as all events processed by
vmx_has_nested_events() require injection, but that will not hold true in
the future, e.g. if there's a pending virtual interrupt in vmcs02.RVI.
I.e. if KVM is trying to deliver a virtual interrupt to L2, the expired
VMX preemption timer will trigger VM-Exit before the virtual interrupt is
delivered, and KVM will effectively hang the vCPU in an endless loop of
forced immediate VM-Exits (because the pending virtual interrupt never
goes away).

Cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

Author: sean-jc

arch/x86/include/asm/kvm_host.h

@@ -1823,7 +1823,7 @@ struct kvm_x86_nested_ops {
 	bool (*is_exception_vmexit)(struct kvm_vcpu *vcpu, u8 vector,
 				    u32 error_code);
 	int (*check_events)(struct kvm_vcpu *vcpu);
-	bool (*has_events)(struct kvm_vcpu *vcpu);
+	bool (*has_events)(struct kvm_vcpu *vcpu, bool for_injection);
 	void (*triple_fault)(struct kvm_vcpu *vcpu);
 	int (*get_state)(struct kvm_vcpu *vcpu,
 			 struct kvm_nested_state __user *user_kvm_nested_state,

arch/x86/kvm/vmx/nested.c

@@ -4032,7 +4032,7 @@ static bool nested_vmx_preemption_timer_pending(struct kvm_vcpu *vcpu)
 	       to_vmx(vcpu)->nested.preemption_timer_expired;
 }
 
-static bool vmx_has_nested_events(struct kvm_vcpu *vcpu)
+static bool vmx_has_nested_events(struct kvm_vcpu *vcpu, bool for_injection)
 {
 	return nested_vmx_preemption_timer_pending(vcpu) ||
 	       to_vmx(vcpu)->nested.mtf_pending;

arch/x86/kvm/x86.c

@@ -10516,7 +10516,7 @@ static int kvm_check_and_inject_events(struct kvm_vcpu *vcpu,
 
 	if (is_guest_mode(vcpu) &&
 	    kvm_x86_ops.nested_ops->has_events &&
-	    kvm_x86_ops.nested_ops->has_events(vcpu))
+	    kvm_x86_ops.nested_ops->has_events(vcpu, true))
 		*req_immediate_exit = true;
 
 	/*
@@ -13157,7 +13157,7 @@ static inline bool kvm_vcpu_has_events(struct kvm_vcpu *vcpu)
 
 	if (is_guest_mode(vcpu) &&
 	    kvm_x86_ops.nested_ops->has_events &&
-	    kvm_x86_ops.nested_ops->has_events(vcpu))
+	    kvm_x86_ops.nested_ops->has_events(vcpu, false))
 		return true;
 
 	if (kvm_xen_has_pending_events(vcpu))