From 2d6cfbaf174b91dfa9a50065f7494634afb39c23 Mon Sep 17 00:00:00 2001 From: Salil Mehta Date: Sun, 3 Nov 2024 10:24:15 +0000 Subject: [PATCH] hw/acpi: Make CPUs ACPI `presence` conditional during vCPU hot-unplug MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On most architectures, during vCPU hot-plug and hot-unplug actions, the firmware or VMM/QEMU can update the OS on vCPU status by toggling the ACPI method `_STA.Present` bit. However, certain CPU architectures prohibit [1] modifications to a CPU’s `presence` status after the kernel has booted. This limitation [2][3] exists because many per-CPU components, such as interrupt controllers and various per-CPU features tightly integrated with CPUs, may not support reconfiguration once the kernel is initialized. Often, these components cannot be powered down, as they may belong to an `always-on` power domain. As a result, some architectures require all CPUs to remain `_STA.Present` after system initialization. Therefore, it is essential to mirror the exact QOM vCPU status through ACPI for the Guest kernel. For this, we should determine—via architecture-specific code[4]—whether vCPUs must always remain present and whether the associated `AcpiCpuStatus::cpu` object should remain valid, even following a vCPU hot-unplug operation. References: [1] Check comment 5 in the bugzilla entry Link: https://bugzilla.tianocore.org/show_bug.cgi?id=4481#c5 [2] KVMForum 2023 Presentation: Challenges Revisited in Supporting Virt CPU Hotplug on architectures that don’t Support CPU Hotplug (like ARM64) a. Kernel Link: https://kvm-forum.qemu.org/2023/KVM-forum-cpu-hotplug_7OJ1YyJ.pdf b. Qemu Link: https://kvm-forum.qemu.org/2023/Challenges_Revisited_in_Supporting_Virt_CPU_Hotplug_-__ii0iNb3.pdf [3] KVMForum 2020 Presentation: Challenges in Supporting Virtual CPU Hotplug on SoC Based Systems (like ARM64) Link: https://kvmforum2020.sched.com/event/eE4m [4] Example implementation of architecture-specific CPU persistence hook Link: https://github.com/salil-mehta/qemu/commit/c0b416b11e5af6505e558866f0eb6c9f3709173e Signed-off-by: Salil Mehta Message-Id: <20241103102419.202225-2-salil.mehta@huawei.com> Reviewed-by: Michael S. Tsirkin Signed-off-by: Michael S. Tsirkin --- hw/acpi/cpu.c | 15 ++++++++++++++- include/hw/core/cpu.h | 1 + 2 files changed, 15 insertions(+), 1 deletion(-) diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c index 5cb60ca8bc..9b03b4292e 100644 --- a/hw/acpi/cpu.c +++ b/hw/acpi/cpu.c @@ -233,6 +233,17 @@ void cpu_hotplug_hw_init(MemoryRegion *as, Object *owner, memory_region_add_subregion(as, base_addr, &state->ctrl_reg); } +static bool should_remain_acpi_present(DeviceState *dev) +{ + CPUClass *k = CPU_GET_CLASS(dev); + /* + * A system may contain CPUs that are always present on one die, NUMA node, + * or socket, yet may be non-present on another simultaneously. Check from + * architecture specific code. + */ + return k->cpu_persistent_status && k->cpu_persistent_status(CPU(dev)); +} + static AcpiCpuStatus *get_cpu_status(CPUHotplugState *cpu_st, DeviceState *dev) { CPUClass *k = CPU_GET_CLASS(dev); @@ -289,7 +300,9 @@ void acpi_cpu_unplug_cb(CPUHotplugState *cpu_st, return; } - cdev->cpu = NULL; + if (!should_remain_acpi_present(dev)) { + cdev->cpu = NULL; + } } static const VMStateDescription vmstate_cpuhp_sts = { diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h index c3ca0babcb..e7de77dc6d 100644 --- a/include/hw/core/cpu.h +++ b/include/hw/core/cpu.h @@ -158,6 +158,7 @@ struct CPUClass { void (*dump_state)(CPUState *cpu, FILE *, int flags); void (*query_cpu_fast)(CPUState *cpu, CpuInfoFast *value); int64_t (*get_arch_id)(CPUState *cpu); + bool (*cpu_persistent_status)(CPUState *cpu); void (*set_pc)(CPUState *cpu, vaddr value); vaddr (*get_pc)(CPUState *cpu); int (*gdb_read_register)(CPUState *cpu, GByteArray *buf, int reg);