[PATCH v2 3/9] ppc/spapr: Use start-powered-off CPUState property

[Thiago Jung Bauermann <bauerman@linux.ibm.com>]

PowerPC sPAPR CPUs start in the halted state, and spapr_reset_vcpu()
attempts to implement this by setting CPUState::halted to 1. But that's too
late for the case of hotplugged CPUs in a machine configure with 2 or more
threads per core.

By then, other parts of QEMU have already caused the vCPU to run in an
unitialized state a couple of times. For example, ppc_cpu_reset() calls
ppc_tlb_invalidate_all(), which ends up calling async_run_on_cpu(). This
kicks the new vCPU while it has CPUState::halted = 0, causing QEMU to issue
a KVM_RUN ioctl on the new vCPU before the guest is able to make the
start-cpu RTAS call to initialize its register state.

This problem doesn't seem to cause visible issues for regular guests, but
on a secure guest running under the Ultravisor it does. The Ultravisor
relies on being able to snoop on the start-cpu RTAS call to map vCPUs to
guests, and this issue causes it to see a stray vCPU that doesn't belong to
any guest.

Fix by setting the start-powered-off CPUState property in
spapr_create_vcpu(), which makes cpu_common_reset() initialize
CPUState::halted to 1 at an earlier moment.

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
---
 hw/ppc/spapr_cpu_core.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

NB: Tested on ppc64le pseries KVM guest with two threads per core. 
Hot-plugging additional cores doesn't cause the bug described above
anymore.

diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
index c4f47dcc04..09feeb5f8f 100644
--- a/hw/ppc/spapr_cpu_core.c
+++ b/hw/ppc/spapr_cpu_core.c
@@ -36,11 +36,6 @@ static void spapr_reset_vcpu(PowerPCCPU *cpu)
 
     cpu_reset(cs);
 
-    /* All CPUs start halted.  CPU0 is unhalted from the machine level
-     * reset code and the rest are explicitly started up by the guest
-     * using an RTAS call */
-    cs->halted = 1;
-
     env->spr[SPR_HIOR] = 0;
 
     lpcr = env->spr[SPR_LPCR];
@@ -288,6 +283,13 @@ static PowerPCCPU *spapr_create_vcpu(SpaprCpuCore *sc, int i, Error **errp)
 
     cpu->machine_data = g_new0(SpaprCpuState, 1);
 
+    /*
+     * All CPUs start halted. CPU0 is unhalted from the machine level reset code
+     * and the rest are explicitly started up by the guest using an RTAS call.
+     */
+    object_property_set_bool(OBJECT(cs), "start-powered-off", true,
+                             &error_abort);
+
     object_unref(obj);
     return cpu;