KVM: x86: Sync the pending Posted-Interrupts

Some Posted-Interrupts from passthrough devices may be lost or
overwritten when the vCPU is in runnable state.

The SN (Suppress Notification) of PID (Posted Interrupt Descriptor) will
be set when the vCPU is preempted (vCPU in KVM_MP_STATE_RUNNABLE state
but not running on physical CPU). If a posted interrupt coming at this
time, the irq remmaping facility will set the bit of PIR (Posted
Interrupt Requests) without ON (Outstanding Notification).
So this interrupt can't be sync to APIC virtualization register and
will not be handled by Guest because ON is zero.

Signed-off-by: Luwei Kang <luwei.kang@intel.com>
[Eliminate the pi_clear_sn fast path. - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -7808,7 +7808,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	 * 1) We should set ->mode before checking ->requests.  Please see
 	 * the comment in kvm_vcpu_exiting_guest_mode().
 	 *
-	 * 2) For APICv, we should set ->mode before checking PIR.ON.  This
+	 * 2) For APICv, we should set ->mode before checking PID.ON. This
 	 * pairs with the memory barrier implicit in pi_test_and_set_on
 	 * (see vmx_deliver_posted_interrupt).
 	 *