[Qemu-devel] [PATCH v4 9/9] s390x/tcg: fix CPU hotplug with single-threaded TCG

[David Hildenbrand <david@redhat.com>]

run_on_cpu() doesn't seem to work reliably until the CPU has been fully
created if the single-threaded TCG main loop is already running.

Therefore, hotplugging a CPU under single-threaded TCG does currently
not work. We should use the direct call instead of going via
run_on_cpu().

So let's use run_on_cpu() for KVM only - KVM requires it due to the initial
CPU reset ioctl. As a nice side effect, we get rid of the ifdef.

Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 target/s390x/cpu.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/target/s390x/cpu.c b/target/s390x/cpu.c
index 40d6980229..271c5ce652 100644
--- a/target/s390x/cpu.c
+++ b/target/s390x/cpu.c
@@ -218,11 +218,18 @@ static void s390_cpu_realizefn(DeviceState *dev, Error **errp)
 #endif
     s390_cpu_gdb_init(cs);
     qemu_init_vcpu(cs);
-#if !defined(CONFIG_USER_ONLY)
-    run_on_cpu(cs, s390_do_cpu_full_reset, RUN_ON_CPU_NULL);
-#else
-    cpu_reset(cs);
-#endif
+
+    /*
+     * KVM requires the initial CPU reset ioctl to be executed on the target
+     * CPU thread. CPU hotplug under single-threaded TCG will not work with
+     * run_on_cpu(), as run_on_cpu() will not work properly if called while
+     * the main thread is already running but the CPU hasn't been realized.
+     */
+    if (kvm_enabled()) {
+        run_on_cpu(cs, s390_do_cpu_full_reset, RUN_ON_CPU_NULL);
+    } else {
+        cpu_reset(cs);
+    }
 
     scc->parent_realize(dev, &err);
 out:
-- 
2.17.1