[Qemu-devel] [RFC PATCH v0 15/15] ppc: Allow hotplugging of CPU cores only

[Bharata B Rao]

pseries kernels support hot-plugging CPUs in core granularity. i,e., when
a core is hotplugged, guest kernel tries to bring up all the threads in
that core. This doesn't match 1-to-1 with QEMU hotplug semantics where
cpu-add monitor command hotplugs 1 CPU at a time.

While it is still an open question as to what kind of CPU hotplug semantics
we should support here, this patch restricts cpu-add monitor command
to allow only CPU cores to be added. All the threads of the hot-plugged
core at onlined at once.

Signed-off-by: Bharata B Rao <bhar...@linux.vnet.ibm.com>
---
 hw/ppc/spapr.c | 27 ++++++++++++++++++++++-----
 1 file changed, 22 insertions(+), 5 deletions(-)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 9a3d1ca..96fb11b 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -1717,7 +1717,7 @@ static int spapr_populate_cpu_dt(CPUState *cs, void *fdt, 
int offset)
     return 0;
 }
 
-static void spapr_cpu_hotplug_add(CPUState *cs)
+static int spapr_cpu_hotplug_add(CPUState *cs)
 {
     int i, j;
     sPAPRDrcEntry *drc_entry;
@@ -1740,6 +1740,11 @@ static void spapr_cpu_hotplug_add(CPUState *cs)
     PowerPCCPUClass *pcc = POWERPC_CPU_GET_CLASS(cs);
     char *nodename;
     int index = ppc_get_vcpu_dt_id(cpu);
+    int smt = kvmppc_smt_threads();
+
+    if ((index % smt) != 0) {
+        return -1;
+    }
 
     drc_entry = spapr_cpu_to_drc_entry(cpu->cpu_dt_id + SPAPR_DRC_CPU_ID_BASE);
     g_assert(drc_entry);
@@ -1797,14 +1802,17 @@ static void spapr_cpu_hotplug_add(CPUState *cs)
     ccs->fdt = fdt;
     ccs->offset_start = offset;
     ccs->state = CC_STATE_PENDING;
+
+    return 0;
 }
 
 static void spapr_cpu_added_req(Notifier *n, void *opaque)
 {
     CPUState *cs = CPU(opaque);
 
-    spapr_cpu_hotplug_add(cs);
-    spapr_cpu_hotplug_add_event(cs);
+    if (!spapr_cpu_hotplug_add(cs)) {
+        spapr_cpu_hotplug_add_event(cs);
+    }
     return;
 }
 
@@ -2193,6 +2201,7 @@ static void ppc_hot_add_cpu(const int64_t id, Error 
**errp)
 {
     CPUState *cs;
     PowerPCCPU *cpu;
+    int i;
 
     if (id < 0) {
         error_setg(errp, "Invalid CPU id: %" PRIi64, id);
@@ -2213,8 +2222,16 @@ static void ppc_hot_add_cpu(const int64_t id, Error 
**errp)
         return;
     }
 
-    cpu = ppc_new_cpu(current_cpu_model);
-    spapr_cpu_reset(cpu);
+    if ((id % smp_threads) != 0) {
+        error_setg(errp, "Invalid CPU id: %" PRIi64
+                   ", only cores can be added", id);
+        return;
+    }
+
+    for (i = 0; i < smp_threads; i++) {
+        cpu = ppc_new_cpu(current_cpu_model);
+        spapr_cpu_reset(cpu);
+    }
 }
 
 static void spapr_machine_class_init(ObjectClass *oc, void *data)
-- 
1.7.11.7