xen: introduce xen_support_evtchn_bind()

[ehem_freebsd_m5p.com]

Summary

xen_vector_callback_enabled is x86 specific and supporting evtchn rebind
on others architectures may be different.

Introduce a new helper to check if we can rebind the event channel.

Note that xen_intr_assign_cpu was only checking if
xen_vector_callback_enabled. Although it looks like to me that rebind
should be supported for PVH.

Submitted by: Elliott Mitchell <[e-mail]>
Original implementation: Julien Grall <[e-mail]>, 2015-10-10 12:40:06

[royger]

I think the commit message needs some kind of adjustments, as it seems to mix event channel rebind with having setup a vector callback for event channel notification delivery?

On x86 you can have a 'global' event channel notification delivery through an IO-APIC pin, which is not per-vcpu. This used to be the old way of signaling event channels to HVM guests until a per-vcpu specific method was introduced using an IDT vector. When using a global IRQ there are event channel operations that make no sense, like binding event channels to specific vCPUs (ie: xen_intr_assign_cpu).

Also you seem to have missed an instance of xen_vector_callback_enabled used in dev/xen/timer/timer.c?

[ehem_freebsd_m5p.com]

Yeah looks like that commit message became garbled. The last paragraph looks like it was a note to self due to being concerned about the conditional in xen_intr_assign_cpu() being distinct from the other spots where xen_support_evtchn_rebind() was used.

For the purpose of this commit, the use of xen_vector_callback_enabled in dev/xen/timer/timer.c is irrelevant as the target (Xen/aarch64) doesn't support the timer device anyway.

[royger]

I would name this xen_percpu_evtchn{_enabled} or some such. Given the current usage I think xen_support_evtchn_rebind is confusing, as it's not about rebinding an event channel, but being able to assign event channels to vCPUs.

[ehem_freebsd_m5p.com]

It is about whether the event channel can be rebound to a different vCPU. I would tend towards xen_has_percpu_evtchn(). This strikes me as fix on commit though. (my main branch on GitLab now has it as xen_has_percpu_evtchn())