Three observations on Bhyve

[symbolics at gmx.com]

Context: Bhyve guest cannot use disc devices through VirtIO due to
interaction with GEOM tasting on the host.

On Fri, Oct 25, 2013 at 05:04:58PM -0700, Peter Grehan wrote:
> >>    That's bhyve colliding with GEOM. There was a thread a while back
> >> about being able to exempt certain block devices from being tasted for
> >> iSCSI LUN usage - bhyve will probably need something similar.
> >
> > Is this different from the kern.geom.notaste sysctl des@ added recently?
> 
>   Yes. I've not tried that, but guessing it could be used as a 
> workaround for something like a zvol-backed image where the zvol is 
> created after the sysctl is set.

I tried this out today. My ada1 device is connected via a eSATA port so
I removed it, set the sysctl and plugged it back in. Of course it didn't
help as the sysctl disables tasting globally so the /dev/ada1 node
doesn't get created either.

>   However, what is really needed is something like a blacklist that can 
> prevent specified devices from being seen by GEOM at all, since the 
> internal structure is only relevant to the guest VM.

I have been looking at the GEOM code and thinking about what sort of
interface I would like. I think I want the ability to tell GEOM not to
taste beyond a named provider.

# geom disk <keyword> ada1

Where <keyword> expands to something like 'notaste'. When this command
is invoked a flag is set on the provider that prevents further tasting
beyond this point and any existing descendent objects are removed. 

I have a little code for this and I will see if I can make it work in a
reasonable way.

I'm CC'ing geom@ because I'd like to know if this approach looks
sensible.

--sym