[PATCH 5.15 090/128] KVM: VMX: Wake vCPU when delivering posted IRQ even if vCPU == this vCPU

[Greg Kroah-Hartman]

From: Sean Christopherson <seanjc@xxxxxxxxxx>

commit fdba608f15e2427419997b0898750a49a735afcb upstream.

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: stable@xxxxxxxxxxxxxxx
Reported-by: Longpeng (Mike) <longpeng2@xxxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
Message-Id: <20211208015236.1616697-18-seanjc@xxxxxxxxxx>
Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/kvm/vmx/vmx.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -3971,8 +3971,7 @@ static int vmx_deliver_posted_interrupt(
 	if (pi_test_and_set_on(&vmx->pi_desc))
 		return 0;
 
-	if (vcpu != kvm_get_running_vcpu() &&
-	    !kvm_vcpu_trigger_posted_interrupt(vcpu, false))
+	if (!kvm_vcpu_trigger_posted_interrupt(vcpu, false))
 		kvm_vcpu_kick(vcpu);
 
 	return 0;