On 6/14/24 9:36 AM, Salil Mehta wrote:
During `machvirt_init()`, QOM ARMCPU objects are pre-created along with the corresponding KVM vCPUs in the host for all possible vCPUs. This is necessary due to the architectural constraint that KVM restricts the deferred creation of KVM vCPUs and VGIC initialization/sizing after VM initialization. Hence, VGIC is pre-sized with possible vCPUs.After the initialization of the machine is complete, the disabled possible KVM vCPUs are parked in the per-virt-machine list "kvm_parked_vcpus," and we release the QOM ARMCPU objects for the disabled vCPUs. These will be re-created when the vCPU is hotplugged again. The QOM ARMCPU object is then re-attached to the corresponding parked KVM vCPU. Alternatively, we could have chosen not to release the QOM CPU objects and kept reusing them. This approach might require some modifications to the `qdevice_add()` interface to retrieve the old ARMCPU object instead of creating a new one for the hotplug request. Each of these approaches has its own pros and cons. This prototype uses the first approach (suggestions are welcome!). Co-developed-by: Keqian Zhu <zhukeqi...@huawei.com> Signed-off-by: Keqian Zhu <zhukeqi...@huawei.com> Signed-off-by: Salil Mehta <salil.me...@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 9d33f30a6a..a72cd3b20d 100644 --- a/hw/arm/virt.c +++ b/hw/arm/virt.c @@ -2050,6 +2050,7 @@ 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; @@ -2111,6 +2112,37 @@ 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)); + } + } + } }
It's probably hard to keep those ARMCPU objects forever. First of all, one vCPU can be hot-added first and then hot-removed afterwards. With those ARMCPU objects kept forever, the syntax of 'device_add' and 'device_del' become broken at least. The ideal mechanism would be to avoid instanciating those ARMCPU objects and destroying them soon. I don't know if ms->possible_cpus->cpus[] can fit and how much efforts needed. Thanks, Gavin
static void virt_cpu_set_properties(Object *cpuobj, const CPUArchId *cpu_slot,
