[Qemu-devel] [PATCH] pseries: Restore support for total vcpus not a multiple of threads-per-core for old machine types

[David Gibson <david@gibson.dropbear.id.au>]

As of pseries-2.7 and later, we require the total number of guest vcpus to
be a multiple of the threads-per-core.  pseries-2.6 and earlier machine
types, however, are supposed to allow this for the sake of migration from
old qemu versions which allowed this.

Unfortunately, 8149e29 "pseries: Enforce homogeneous threads-per-core"
broke this by not considering the old machine type case.  This fixes it by
only applying the check when the machine type supports hotpluggable cpus.
By not-entirely-coincidence, that corresponds to the same time when we
started enforcing total threads being a multiple of threads-per-core.

Fixes: 8149e2992f7811355cc34721b79d69d1a3a667dd

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 hw/ppc/spapr.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index c92d269..bcb0e18 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -2863,7 +2863,13 @@ static void spapr_core_pre_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
         goto out;
     }
 
-    if (cc->nr_threads != smp_threads) {
+    /*
+     * In general we should have homogeneous threads-per-core, but old
+     * (pre hotplug support) machine types allow the last core to have
+     * reduced threads as a compatibility hack for when we allowed
+     * total vcpus not a multiple of threads-per-core.
+     */
+    if (mc->has_hotpluggable_cpus && (cc->nr_threads != smp_threads)) {
         error_setg(errp, "invalid nr-threads %d, must be %d",
                    cc->nr_threads, smp_threads);
         return;
-- 
2.9.4