Re: Mentor of the Project "PCI frontend XEN PV driver"?

Justin T. Gibbs gibbs at
Tue Apr 16 14:27:47 UTC 2013

On Apr 12, 2013, at 1:21 AM, David Chisnall <theraven at> wrote:

> On 12 Apr 2013, at 05:13, "Justin T. Gibbs" <gibbs at> wrote:
>> This particular project was proposed for last year's GSoC when both attilio at and I were actively working on full Xen PV support for FreeBSD.  That project still continues, but at a slower pace due to changing needs at our respective employers.  Given the lack of maturity of our Xen PV port, I think this would be a difficult project to attempt at this time.
>> If you are interested in Xen and have access to x86 hardware with HVM capabilities, writing a SCSI or USB PV frontend for FreeBSD is a better project to select. FreeBSD's Xen HVM support is very robust and, unlike the PCI frontend driver, these drivers can be developed using an HVM guest.  The resulting drivers should work equally well in full PV mode (with perhaps a few minor tweaks) when it is better supported.
>> I will update the wiki sometime tomorrow with more information on Xen projects that are appropriate for this year's GSoC.
> Do we support PHV mode in Xen yet?  If not, then this might be a good GSoC project.  Most of the effort - it uses the PV boot mechanism, but then the HVM pmap and PV drivers (including event channels, no APIC emulation).  Unlike the PV mode, it won't require us to maintain two i386 pmap implementations for the increasingly rare case of trying to run FreeBSD VMs on CPUs without virtualisation support, and it doesn't have the QEMU overhead in dom0 that HVM does (even when you're using fully PV devices, you still need it to boot), and which causes hosting providers to charge more for HVM.
> David

The last time I was really active in the Xen community (over a year ago), PHV was still in heavy development.  However, it is certainly coming.  Once Cherry has full PV working for us, we should certainly start the process of supporting it.


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