qemu/arm-virt-Release-objects-for-disabled-possible-vCPUs.patch

From 097e3b46a7eede0182a846f7b993e14d3eed83b7 Mon Sep 17 00:00:00 2001
From: Salil Mehta <salil.mehta@huawei.com>
Date: Tue, 9 Jun 2020 03:01:08 +0100
Subject: [PATCH] arm/virt: Release objects for *disabled* possible vCPUs after
 init

During machvirt_init(), QOM ARMCPU objects are also pre-created along with the
corresponding KVM vCPUs in the host for all possible vCPUs. This necessary
because of the architectural constraint, KVM restricts the deferred creation of
the KVM vCPUs and VGIC initialization/sizing after VM init. Hence, VGIC is
pre-sized with possible vCPUs.

After initialization of the machine is complete disabled possible KVM vCPUs are
then parked at the per-virt-machine list "kvm_parked_vcpus" and we release the
QOM ARMCPU objects for the disabled vCPUs. These shall be re-created at the time
when vCPU is hotplugged again. QOM ARMCPU object is then re-attached with
corresponding parked KVM vCPU.

Alternatively, we could've never released the QOM CPU objects and kept on
reusing. This approach might require some modifications of qdevice_add()
interface to get old ARMCPU object instead of creating a new one for the hotplug
request.

Each of the above approaches come with their own pros and cons. This prototype
uses the 1st approach.(suggestions are welcome!)

Co-developed-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Co-developed-by: Keqian Zhu <zhukeqian1@huawei.com>
Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
---
 hw/arm/virt.c | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 91b2653c03..bf385a469c 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -2060,6 +2060,7 @@ static void virt_cpu_post_init(VirtMachineState *vms, MemoryRegion *sysmem)
 {
     CPUArchIdList *possible_cpus = vms->parent.possible_cpus;
     int max_cpus = MACHINE(vms)->smp.max_cpus;
+    MachineState *ms = MACHINE(vms);
     bool aarch64, steal_time;
     CPUState *cpu;
     int n;
@@ -2120,6 +2121,37 @@ static void virt_cpu_post_init(VirtMachineState *vms, MemoryRegion *sysmem)
             }
         }
     }
+
+    if (kvm_enabled() || tcg_enabled()) {
+        for (n = 0; n < possible_cpus->len; n++) {
+            cpu = qemu_get_possible_cpu(n);
+
+            /*
+             * Now, GIC has been sized with possible CPUs and we dont require
+             * disabled vCPU objects to be represented in the QOM. Release the
+             * disabled ARMCPU objects earlier used during init for pre-sizing.
+             *
+             * We fake to the guest through ACPI about the presence(_STA.PRES=1)
+             * of these non-existent vCPUs at VMM/qemu and present these as
+             * disabled vCPUs(_STA.ENA=0) so that they cant be used. These vCPUs
+             * can be later added to the guest through hotplug exchanges when
+             * ARMCPU objects are created back again using 'device_add' QMP
+             * command.
+             */
+            /*
+             * RFC: Question: Other approach could've been to keep them forever
+             * and release it only once when qemu exits as part of finalize or
+             * when new vCPU is hotplugged. In the later old could be released
+             * for the newly created object for the same vCPU?
+             */
+            if (!qemu_enabled_cpu(cpu)) {
+                CPUArchId *cpu_slot;
+                cpu_slot = virt_find_cpu_slot(ms, cpu->cpu_index);
+                cpu_slot->cpu = NULL;
+                object_unref(OBJECT(cpu));
+            }
+        }
+    }
 }
 
 static void virt_cpu_set_properties(Object *cpuobj, const CPUArchId *cpu_slot,
-- 
2.27.0