Paul Mackerras [off-list ref] writes:

On Sun, Jun 29, 2014 at 04:47:31PM +0530, Aneesh Kumar K.V wrote:

quoted

This makes it consistent with h_enter where we clear the key
bits. We also want to use virtual page class key protection mechanism
for indicating host page fault. For that we will be using key class
index 30 and 31. So prevent the guest from updating key bits until
we add proper support for virtual page class protection mechanism for
the guest. This will not have any impact for PAPR linux guest because
Linux guest currently don't use virtual page class key protection model

As things stand, without this patch series, we do actually have
everything we need in the kernel for guests to use virtual page class
keys.  Arguably we should have a capability to tell userspace how many
storage keys the guest can use, but that's the only missing piece as
far as I can see.

yes.

If we add such a capability, I can't see any reason why we should need
to disable guest use of storage keys in this patchset.

With this patchset, we would need additonal changes to find out whether the key
fault happened because of the guest's usage of the key. I was planning to do
that as an add-on series to keep the changes in this minimal. Also since
linux didn't use keys i was not sure whether guest support of keys is an
important item.

-aneesh