Difference in event channel implementation for Xen PV vs HVM guests

[Roger Pau Monné]

Hello,

While working on improving XENHVM (I've been looking at adding PV
timers), I've realized that the event channel implementation in PV vs
HVM mode differs greatly. Xen PV port uses sys/xen/evtchn/evtchn.c while
Xen HVM uses sys/dev/xenpci/evtchn.c, and the Xen HVM implementation is
greatly reduced (only contains the necessary functions to operate
backends/frontends).

To implement PV timers I need to expand the event channel interface for
XENHVM, and I was wondering why FreeBSD choose to have two different
implementations, the main difference between PV and HVM is the event
callback, but I guess this can be abstracted between the two different
implementations, and then everything else could be reused. Am I missing
something obvious?

Is there any known technical problem in modifying XENHVM to use the full
event channel implementation present in sys/xen/evtchn/evtchn.c that
prevented XENHVM from using it in the first place?

(Sorry if I've Cc'ed someone not related)

Thanks, Roger.