KVM: PPC: Book3S HV: Don't dynamically split core when already split

In static micro-threading modes, the dynamic micro-threading code
is supposed to be disabled, because subcores can't make independent
decisions about what micro-threading mode to put the core in - there is
only one micro-threading mode for the whole core.  The code that
implements dynamic micro-threading checks for this, except that the
check was missed in one case.  This means that it is possible for a
subcore in static 2-way micro-threading mode to try to put the core
into 4-way micro-threading mode, which usually leads to stuck CPUs,
spinlock lockups, and other stalls in the host.

The problem was in the can_split_piggybacked_subcores() function, which
should always return false if the system is in a static micro-threading
mode.  This fixes the problem by making can_split_piggybacked_subcores()
use subcore_config_ok() for its checks, as subcore_config_ok() includes
the necessary check for the static micro-threading modes.

Credit to Gautham Shenoy for working out that the reason for the hangs
and stalls we were seeing was that we were trying to do dynamic 4-way
micro-threading while we were in static 2-way mode.

Fixes: b4deba5c41
Cc: vger@stable.kernel.org # v4.3
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kvm/book3s_hv.c

@@ -2060,7 +2060,7 @@ static bool can_split_piggybacked_subcores(struct core_info *cip)
 			return false;
 		n_subcores += (cip->subcore_threads[sub] - 1) >> 1;
 	}
-	if (n_subcores > 3 || large_sub < 0)
+	if (large_sub < 0 || !subcore_config_ok(n_subcores + 1, 2))
 		return false;
 
 	/*