spapr: Handle HPT allocation failure in nested guest

The nested KVM code does not yet support HPT guests. Calling the
KVM_CAP_PPC_ALLOC_HTAB ioctl currently leads to KVM setting the guest
as HPT and erroneously executing code in L1 that should only run in
hypervisor mode, leading to an exception in the L1 vcpu thread when it
enters the nested guest.

This can be reproduced with -machine max-cpu-compat=power8 in the L2
guest command line.

The KVM code has since been modified to fail the ioctl when running in
a nested environment so QEMU needs to be able to handle that. This
patch provides an error message informing the user about the lack of
support for HPT in nested guests.

Reported-by: Satheesh Rajendran <sathnaga@linux.vnet.ibm.com>
Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
Message-Id: <20200911043123.204162-1-farosas@linux.ibm.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr.c

@@ -1483,6 +1483,12 @@ void spapr_reallocate_hpt(SpaprMachineState *spapr, int shift,
     spapr_free_hpt(spapr);
 
     rc = kvmppc_reset_htab(shift);
+
+    if (rc == -EOPNOTSUPP) {
+        error_setg(errp, "HPT not supported in nested guests");
+        return;
+    }
+
     if (rc < 0) {
         /* kernel-side HPT needed, but couldn't allocate one */
         error_setg_errno(errp, errno,