spapr/xive: Sanity checks of OV5 during CAS

spapr/xive: Sanity checks of OV5 during CAS

If a machine is started with ic-mode=xive but the guest only knows
about XICS, eg. an RHEL 7.6 guest, the kernel panics. This is
expected but a bit unfortunate since the crash doesn't provide
much information for the end user to guess what's happening.

Detect that during CAS and exit QEMU with a proper error message
instead, like it is already done for the MMU.

Even if this is less likely to happen, the opposite case of a guest
that only knows about XIVE would certainly fail all the same if the
machine is started with ic-mode=xics.

Also, the only valid values a guest can pass in byte 23 of OV5 during
CAS are 0b00 (XIVE legacy mode) and 0b01 (XIVE exploitation mode). Any
other value is a bug, at least with the current spec. Again, it does
not seem right to let the guest go on without a precise idea of the
interrupt mode it asked for.

Handle these cases as well.

Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Signed-off-by: Greg Kurz <groug@kaod.org>
Message-Id: <155793986451.464434.12887933000007255549.stgit@bahia.lan>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>