KVM: x86/xen: remove WARN_ON_ONCE() with false positives in evtchn delivery

The kvm_xen_inject_vcpu_vector() function has a comment saying "the fast
version will always work for physical unicast", justifying its use of
kvm_irq_delivery_to_apic_fast() and the WARN_ON_ONCE() when that fails.

In fact that assumption isn't true if X2APIC isn't in use by the guest
and there is (8-bit x)APIC ID aliasing. A single "unicast" destination
APIC ID *may* then be delivered to multiple vCPUs. Remove the warning,
and in fact it might as well just call kvm_irq_delivery_to_apic().

Reported-by: Michal Luczaj <mhal@rbox.co>
Fixes: fde0451 ("KVM: x86/xen: Support per-vCPU event channel upcall via local APIC")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Paul Durrant <paul@xen.org>
Link: https://lore.kernel.org/r/20240227115648.3104-4-dwmw2@infradead.org
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: dwmw2

arch/x86/kvm/xen.c

@@ -10,7 +10,7 @@
 #include "x86.h"
 #include "xen.h"
 #include "hyperv.h"
-#include "lapic.h"
+#include "irq.h"
 
 #include <linux/eventfd.h>
 #include <linux/kvm_host.h>
@@ -570,7 +570,6 @@ void kvm_xen_update_runstate(struct kvm_vcpu *v, int state)
 void kvm_xen_inject_vcpu_vector(struct kvm_vcpu *v)
 {
 	struct kvm_lapic_irq irq = { };
-	int r;
 
 	irq.dest_id = v->vcpu_id;
 	irq.vector = v->arch.xen.upcall_vector;
@@ -579,8 +578,7 @@ void kvm_xen_inject_vcpu_vector(struct kvm_vcpu *v)
 	irq.delivery_mode = APIC_DM_FIXED;
 	irq.level = 1;
 
-	/* The fast version will always work for physical unicast */
-	WARN_ON_ONCE(!kvm_irq_delivery_to_apic_fast(v->kvm, NULL, &irq, &r, NULL));
+	kvm_irq_delivery_to_apic(v->kvm, NULL, &irq, NULL);
 }
 
 /*