KVM: VMX: Wake vCPU when delivering posted IRQ even if vCPU == this vCPU

KVM: VMX: Wake vCPU when delivering posted IRQ even if vCPU == this vCPU

Drop a check that guards triggering a posted interrupt on the currently
running vCPU, and more importantly guards waking the target vCPU if
triggering a posted interrupt fails because the vCPU isn't IN_GUEST_MODE.
If a vIRQ is delivered from asynchronous context, the target vCPU can be
the currently running vCPU and can also be blocking, in which case
skipping kvm_vcpu_wake_up() is effectively dropping what is supposed to
be a wake event for the vCPU.

The "do nothing" logic when "vcpu == running_vcpu" mostly works only
because the majority of calls to ->deliver_posted_interrupt(), especially
when using posted interrupts, come from synchronous KVM context.  But if
a device is exposed to the guest using vfio-pci passthrough, the VFIO IRQ
and vCPU are bound to the same pCPU, and the IRQ is _not_ configured to
use posted interrupts, wake events from the device will be delivered to
KVM from IRQ context, e.g.

  vfio_msihandler()
  |
  |-> eventfd_signal()
      |
      |-> ...
          |
          |->  irqfd_wakeup()
               |
               |->kvm_arch_set_irq_inatomic()
                  |
                  |-> kvm_irq_delivery_to_apic_fast()
                      |
                      |-> kvm_apic_set_irq()

This also aligns the non-nested and nested usage of triggering posted
interrupts, and will allow for additional cleanups.

Fixes: 379a3c8ee444 ("KVM: VMX: Optimize posted-interrupt delivery for timer fastpath")
Cc: [email protected]
Reported-by: Longpeng (Mike) <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Reviewed-by: Maxim Levitsky <[email protected]>
Message-Id: <20211208015236.1616697[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>