Difference in event channel implementation for Xen PV vs HVM guests

Justin T. Gibbs gibbs at freebsd.org
Mon Mar 18 13:08:53 UTC 2013

On Mar 18, 2013, at 6:35 AM, Roger Pau Monné <roger.pau at citrix.com> wrote:

> 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.
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Hi Roger,

I know of no reasons why XENHVM cannot use the full event channel
interface.  In fact, Spectra Logic implemented PV timers and a
general cleanup of the HVM event channel interface.  I haven't merged
it back yet because I know the changes break PV and I haven't found
time to clean up the PV code, merge the HVM and PV event channel
systems, and move IPI/MSI delivery to event channels.

I've uploaded Spectra's changes here:


The diffs file provides the history of the original checkins to our
Perforce repository.  The tar file includes all the files that have
been modified and reflects our efforts to keep our code base in
sync with stable/9.  Apart from the PV issues outlined above, I
would expect the code provided to just drop right in to stable/9.

Unfortunately, Xen support is not a current priority for Spectra
so I don't have a lot of day job time to focus on getting this code back
into FreeBSD.  However, if this code looks like it would suite
your needs, and you have resources for testing i386/PV, I'd be happy
to collaborate with you and will make the time to help get this


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