[PATCH v2 4/4] target/ppc: handle vcpu hotplug failure gracefully

[Harsh Prateek Bora <harshpb@linux.ibm.com>]

On ppc64, the PowerVM hypervisor runs with limited memory and a VCPU
creation during hotplug may fail during kvm_ioctl for KVM_CREATE_VCPU,
leading to termination of guest since errp is set to &error_fatal while
calling kvm_init_vcpu. This unexpected behaviour can be avoided by
pre-creating and parking vcpu on success or return error otherwise.
This enables graceful error delivery for any vcpu hotplug failures while
the guest can keep running.

Based on api refactoring to create/park vcpus introduced in 1/8 of patch series:
https://lore.kernel.org/qemu-devel/20240312020000.12992-2-salil.mehta@huawei.com/

Tested OK by repeatedly doing a hotplug/unplug of vcpus as below:

 #virsh setvcpus hotplug 40
 #virsh setvcpus hotplug 70
error: internal error: unable to execute QEMU command 'device_add':
kvmppc_cpu_realize: vcpu hotplug failed with -12

Reported-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
Suggested-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
Suggested-by: Vaibhav Jain <vaibhav@linux.ibm.com>
Signed-off by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Tested-by: Anushree Mathur <anushree.mathur@linux.vnet.ibm.com>
---
 target/ppc/kvm.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/target/ppc/kvm.c b/target/ppc/kvm.c
index 63930d4a77..25f0cf0ba8 100644
--- a/target/ppc/kvm.c
+++ b/target/ppc/kvm.c
@@ -48,6 +48,8 @@
 #include "qemu/mmap-alloc.h"
 #include "elf.h"
 #include "sysemu/kvm_int.h"
+#include "sysemu/kvm.h"
+#include "hw/core/accel-cpu.h"
 
 #define PROC_DEVTREE_CPU      "/proc/device-tree/cpus/"
 
@@ -2339,6 +2341,26 @@ static void alter_insns(uint64_t *word, uint64_t flags, bool on)
     }
 }
 
+static bool kvmppc_cpu_realize(CPUState *cs, Error **errp)
+{
+    int ret;
+
+    cs->cpu_index = cpu_get_free_index();
+
+    POWERPC_CPU(cs)->vcpu_id = cs->cpu_index;
+
+    if (cs->parent_obj.hotplugged) {
+        /* create and park to fail gracefully in case vcpu hotplug fails */
+        ret = kvm_create_and_park_vcpu(cs);
+        if (ret) {
+            error_setg(errp, "%s: vcpu hotplug failed with %d",
+                             __func__, ret);
+            return false;
+        }
+    }
+    return true;
+}
+
 static void kvmppc_host_cpu_class_init(ObjectClass *oc, void *data)
 {
     PowerPCCPUClass *pcc = POWERPC_CPU_CLASS(oc);
@@ -2958,4 +2980,6 @@ void kvmppc_set_reg_tb_offset(PowerPCCPU *cpu, int64_t tb_offset)
 
 void kvm_arch_accel_class_init(ObjectClass *oc)
 {
+    AccelClass *ac = ACCEL_CLASS(oc);
+    ac->cpu_common_realize = kvmppc_cpu_realize;
 }
-- 
2.39.3