Hello! This series adds what looks like cpuhotplug support to arm64 for use in virtual machines. It does this by moving the cpu_register() calls for architectures that support ACPI out of the arch code by using GENERIC_CPU_DEVICES, then into the ACPI processor driver. The kubernetes folk really want to be able to add CPUs to an existing VM, in exactly the same way they do on x86. The use-case is pre-booting guests with one CPU, then adding the number that were actually needed when the workload is provisioned. Wait? Doesn't arm64 support cpuhotplug already!? In the arm world, cpuhotplug gets used to mean removing the power from a CPU. The CPU is offline, and remains present. For x86, and ACPI, cpuhotplug has the additional step of physically removing the CPU, so that it isn't present anymore. Arm64 doesn't support this, and can't support it: CPUs are really a slice of the SoC, and there is not enough information in the existing ACPI tables to describe which bits of the slice also got removed. Without a reference machine: adding this support to the spec is a wild goose chase. Critically: everything described in the firmware tables must remain present. For a virtual machine this is easy as all the other bits of 'virtual SoC' are emulated, so they can (and do) remain present when a vCPU is 'removed'. On a system that supports cpuhotplug the MADT has to describe every possible CPU at boot. Under KVM, the vGIC needs to know about every possible vCPU before the guest is started. With these constraints, virtual-cpuhotplug is really just a hypervisor/firmware policy about which CPUs can be brought online. This series adds support for virtual-cpuhotplug as exactly that: firmware policy. This may even work on a physical machine too; for a guest the part of firmware is played by the VMM. (typically Qemu). PSCI support is modified to return 'DENIED' if the CPU can't be brought online/enabled yet. The CPU object's _STA method's enabled bit is used to indicate firmware's current disposition. If the CPU has its enabled bit clear, it will not be registered with sysfs, and attempts to bring it online will fail. The notifications that _STA has changed its value then work in the same way as physical hotplug, and firmware can cause the CPU to be registered some time later, allowing it to be brought online. This creates something that looks like cpuhotplug to user-space, as the sysfs files appear and disappear, and the udev notifications look the same. One notable difference is the CPU present mask, which is exposed via sysfs. Because the CPUs remain present throughout, they can still be seen in that mask. This value does get used by webbrowsers to estimate the number of CPUs as the CPU online mask is constantly changed on mobile phones. Linux is tolerant of PSCI returning errors, as its always been allowed to do that. To avoid confusing OS that can't tolerate this, we needed an additional bit in the MADT GICC flags. This series copies ACPI_MADT_ONLINE_CAPABLE, which appears to be for this purpose, but calls it ACPI_MADT_GICC_CPU_CAPABLE as it has a different bit position in the GICC. This code is unconditionally enabled for all ACPI architectures. If there are problems with firmware tables on some devices, the CPUs will already be online by the time the acpi_processor_make_enabled() is called. A mismatch here causes a firmware-bug message and kernel taint. This should only affect people with broken firmware who also boot with maxcpus=1, and bring CPUs online later. I had a go at switching the remaining architectures over to GENERIC_CPU_DEVICES, so that the Kconfig symbol can be removed, but I got stuck with powerpc and s390. The first patch has already been posted as a fix here: https://www.spinics.net/lists/linux-ia64/msg21920.html I've only build tested Loongarch and ia64. If folk want to play along at home, you'll need a copy of Qemu that supports this. https://github.com/salil-mehta/qemu.git salil/virt-cpuhp-armv8/rfc-v1-port29092022.psci.present You'll need to fix the numbers of KVM_CAP_ARM_HVC_TO_USER and KVM_CAP_ARM_PSCI_TO_USER to match your host kernel. Replace your '-smp' argument with something like: | -smp cpus=1,maxcpus=3,cores=3,threads=1,sockets=1 then feed the following to the Qemu montior; | (qemu) device_add driver=host-arm-cpu,core-id=1,id=cpu1 | (qemu) device_del cpu1 This series is based on v6.2-rc3, and can be retrieved from: https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/ virtual_cpu_hotplug/rfc/v1 Thanks, James Morse (29): ia64: Fix build error due to switch case label appearing next to declaration ACPI: Move ACPI_HOTPLUG_CPU to be enabled per architecture drivers: base: Use present CPUs in GENERIC_CPU_DEVICES drivers: base: Allow parts of GENERIC_CPU_DEVICES to be overridden drivers: base: Move cpu_dev_init() after node_dev_init() arm64: setup: Switch over to GENERIC_CPU_DEVICES using arch_register_cpu() ia64/topology: Switch over to GENERIC_CPU_DEVICES x86/topology: Switch over to GENERIC_CPU_DEVICES LoongArch: Switch over to GENERIC_CPU_DEVICES arch_topology: Make register_cpu_capacity_sysctl() tolerant to late CPUs ACPI: processor: Add support for processors described as container packages ACPI: processor: Register CPUs that are online, but not described in the DSDT ACPI: processor: Register all CPUs from acpi_processor_get_info() ACPI: Rename ACPI_HOTPLUG_CPU to include 'present' ACPI: Move acpi_bus_trim_one() before acpi_scan_hot_remove() ACPI: Rename acpi_processor_hotadd_init and remove pre-processor guards ACPI: Add post_eject to struct acpi_scan_handler for cpu hotplug ACPI: Check _STA present bit before making CPUs not present ACPI: Warn when the present bit changes but the feature is not enabled drivers: base: Implement weak arch_unregister_cpu() LoongArch: Use the __weak version of arch_unregister_cpu() arm64: acpi: Move get_cpu_for_acpi_id() to a header ACPICA: Add new MADT GICC flags fields [code first?] arm64, irqchip/gic-v3, ACPI: Move MADT GICC enabled check into a helper irqchip/gic-v3: Don't return errors from gic_acpi_match_gicc() irqchip/gic-v3: Add support for ACPI's disabled but 'online capable' CPUs ACPI: add support to register CPUs based on the _STA enabled bit arm64: document virtual CPU hotplug's expectations cpumask: Add enabled cpumask for present CPUs that can be brought online Jean-Philippe Brucker (3): arm64: psci: Ignore DENIED CPUs KVM: arm64: Pass hypercalls to userspace KVM: arm64: Pass PSCI calls to userspace Documentation/arm64/cpu-hotplug.rst | 79 ++++++++++++ Documentation/arm64/index.rst | 1 + Documentation/virt/kvm/api.rst | 31 ++++- Documentation/virt/kvm/arm/hypercalls.rst | 1 + arch/arm64/Kconfig | 1 + arch/arm64/include/asm/acpi.h | 11 ++ arch/arm64/include/asm/cpu.h | 1 - arch/arm64/include/asm/kvm_host.h | 2 + arch/arm64/kernel/acpi_numa.c | 11 -- arch/arm64/kernel/psci.c | 2 +- arch/arm64/kernel/setup.c | 13 +- arch/arm64/kernel/smp.c | 5 +- arch/arm64/kvm/arm.c | 15 ++- arch/arm64/kvm/hypercalls.c | 28 ++++- arch/arm64/kvm/psci.c | 13 ++ arch/ia64/Kconfig | 2 + arch/ia64/include/asm/acpi.h | 2 +- arch/ia64/include/asm/cpu.h | 11 -- arch/ia64/kernel/acpi.c | 6 +- arch/ia64/kernel/setup.c | 2 +- arch/ia64/kernel/sys_ia64.c | 7 +- arch/ia64/kernel/topology.c | 35 +----- arch/loongarch/Kconfig | 2 + arch/loongarch/kernel/topology.c | 31 +---- arch/x86/Kconfig | 2 + arch/x86/include/asm/cpu.h | 6 - arch/x86/kernel/acpi/boot.c | 4 +- arch/x86/kernel/topology.c | 19 +-- drivers/acpi/Kconfig | 5 +- drivers/acpi/acpi_processor.c | 146 +++++++++++++++++----- drivers/acpi/processor_core.c | 2 +- drivers/acpi/scan.c | 116 +++++++++++------ drivers/base/arch_topology.c | 38 ++++-- drivers/base/cpu.c | 31 ++++- drivers/base/init.c | 2 +- drivers/firmware/psci/psci.c | 2 + drivers/irqchip/irq-gic-v3.c | 38 +++--- include/acpi/acpi_bus.h | 1 + include/acpi/actbl2.h | 1 + include/kvm/arm_hypercalls.h | 1 + include/kvm/arm_psci.h | 4 + include/linux/acpi.h | 10 +- include/linux/cpu.h | 6 + include/linux/cpumask.h | 25 ++++ include/uapi/linux/kvm.h | 2 + kernel/cpu.c | 3 + 46 files changed, 532 insertions(+), 244 deletions(-) create mode 100644 Documentation/arm64/cpu-hotplug.rst -- 2.30.2
