[Qemu-devel] [RFC PATCH v4 09/11] spapr: Support topologies with unfilled cores

[Bharata B Rao <bharata@linux.vnet.ibm.com>]

QEMU currently supports CPU topologies where there can be cores
which are not completely filled with all the threads as per the
specifed SMT mode.

Restore support for such topologies (Eg -smp 15,cores=4,threads=4)
The last core will always have the deficit even when -device options are
used to cold-plug the cores.

Signed-off-by: Bharata B Rao <bharata@linux.vnet.ibm.com>
---
 hw/ppc/spapr.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 74637b3..004a8e1 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -94,6 +94,8 @@
 
 #define HTAB_SIZE(spapr)        (1ULL << ((spapr)->htab_shift))
 
+static int smp_remaining_cpus;
+
 static XICSState *try_create_xics(const char *type, int nr_servers,
                                   int nr_irqs, Error **errp)
 {
@@ -1700,6 +1702,7 @@ static void ppc_spapr_init(MachineState *machine)
     int smp_max_cores = DIV_ROUND_UP(max_cpus, smp_threads);
     int smp_cores = DIV_ROUND_UP(smp_cpus, smp_threads);
 
+    smp_remaining_cpus = smp_cpus;
     msi_supported = true;
 
     QLIST_INIT(&spapr->phbs);
@@ -2202,6 +2205,7 @@ static void spapr_cpu_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
     Error *local_err = NULL;
     void *fdt = NULL;
     int i, fdt_offset = 0;
+    int threads_per_core;
 
     /* Set NUMA node for the added CPUs  */
     for (i = 0; i < nb_numa_nodes; i++) {
@@ -2224,8 +2228,22 @@ static void spapr_cpu_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
         return;
     }
 
+    /* Create SMT threads of the core.
+     *
+     * Support topologies like -smp 15,cores=4,threads=4 where one core
+     * will have less than the specified SMT threads. The last core will
+     * always have the deficit even when -device options are used to
+     * cold-plug the cores.
+     */
+    if ((smp_remaining_cpus > 0) && (smp_remaining_cpus < smp_threads)) {
+        threads_per_core = smp_remaining_cpus;
+    } else {
+        threads_per_core = smp_threads;
+    }
+    smp_remaining_cpus -= threads_per_core;
+
     /* Create SMT threads of the core. */
-    for (i = 1; i < smp_threads; i++) {
+    for (i = 1; i < threads_per_core; i++) {
         cpu = cpu_ppc_init(current_machine->cpu_model);
         if (!cpu) {
             error_report("Unable to find PowerPC CPU definition: %s",
-- 
2.1.0