Re: [PATCH v7 26/51] hw/xen: Add xen_evtchn device for event channel emulation

[Paul Durrant]

On 17/01/2023 11:02, David Woodhouse wrote:
> On Tue, 2023-01-17 at 10:56 +0000, Paul Durrant wrote:
> > I'm just having a hard time seeing why passing 0 to
> > xen_evtchn_set_callback_param() does anything useful...
> >
> > +    switch (param >> CALLBACK_VIA_TYPE_SHIFT) {
> > +    case HVM_PARAM_CALLBACK_TYPE_VECTOR: {
> > +        struct kvm_xen_hvm_attr xa = {
> > +            .type = KVM_XEN_ATTR_TYPE_UPCALL_VECTOR,
> > +            .u.vector = (uint8_t)param,
> > +        };
> >
> > HVM_PARAM_CALLBACK_TYPE_VECTOR is 2 AFAICT, so it won't hit that case.
> > Also, you appear to be passing the unshifted param to kernel anyway.
> >
> > What is the call trying to achieve?
>
> Zero is HVM_PARAM_CALLBACK_TYPE_GSI, with GSI==0. It's basically
> disabling event channel delivery for the new kernel.

AFAICT it is doing nothing at this point. Unless I am going insane it 
results in this codepath:

+    default:
+        ret = -ENOSYS;
+        break;
+    }
+
+    if (!ret) {
+        s->callback_param = param;
+        s->evtchn_in_kernel = in_kernel;
+    }

So it doesn't result in any cleanup. What am I missing?

  Paul