KVM: x86: Exempt pending triple fault from event injection sanity check

Exempt pending triple faults, a.k.a. KVM_REQ_TRIPLE_FAULT, when asserting
that KVM didn't attempt to queue a new exception during event injection.
KVM needs to emulate the injection itself when emulating Real Mode due to
lack of unrestricted guest support (VMX) and will queue a triple fault if
that emulation fails.

Ideally the assertion would more precisely filter out the emulated Real
Mode triple fault case, but rmode.vm86_active is buried in vcpu_vmx and
can't be queried without a new kvm_x86_ops.  And unlike "regular"
exceptions, triple fault cannot put the vCPU into an infinite loop; the
triple fault will force either an exit to userspace or a nested VM-Exit,
and triple fault after nested VM-Exit will force an exit to userspace.
I.e. there is no functional issue, so just suppress the warning for
triple faults.

Opportunistically convert the warning to a one-time thing, when it
fires, it fires _a lot_, and is usually user triggerable, i.e. can be
used to spam the kernel log.

Fixes: 7055fb11 ("KVM: x86: Treat pending TRIPLE_FAULT requests as pending exceptions")
Reported-by: kernel test robot <yujie.liu@intel.com>
Link: https://lore.kernel.org/r/202209301338.aca913c3-yujie.liu@intel.comSigned-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220930230008.1636044-1-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -10044,7 +10044,20 @@ static int kvm_check_and_inject_events(struct kvm_vcpu *vcpu,
 	    kvm_x86_ops.nested_ops->has_events(vcpu))
 		*req_immediate_exit = true;
 
-	WARN_ON(kvm_is_exception_pending(vcpu));
+	/*
+	 * KVM must never queue a new exception while injecting an event; KVM
+	 * is done emulating and should only propagate the to-be-injected event
+	 * to the VMCS/VMCB.  Queueing a new exception can put the vCPU into an
+	 * infinite loop as KVM will bail from VM-Enter to inject the pending
+	 * exception and start the cycle all over.
+	 *
+	 * Exempt triple faults as they have special handling and won't put the
+	 * vCPU into an infinite loop.  Triple fault can be queued when running
+	 * VMX without unrestricted guest, as that requires KVM to emulate Real
+	 * Mode events (see kvm_inject_realmode_interrupt()).
+	 */
+	WARN_ON_ONCE(vcpu->arch.exception.pending ||
+		     vcpu->arch.exception_vmexit.pending);
 	return 0;
 
 out: