This patch set changes hands again in an attempt to set a new record for most people who have worked on a single problem. Miguel has been working on a rename and factoring out of arch specific code patch set that will clash with this. https://lore.kernel.org/linux-acpi/20240409150536.9933-1-miguel.luis@xxxxxxxxxx/ [RFC PATCH 0/4] ACPI: processor: refactor acpi_processor_{get_info|remove} v5 changes: - Rebase on Rafael's rework of acpi_scan_check_and_detach() series that superceeded the original first patch. https://lore.kernel.org/linux-acpi/6021126.lOV4Wx5bFT@kreacher/ That dealt with what I thought was the most controversial part of the series - checking the enabled bit ACPI _STA for CPUS. - Change the overall handling so that arch_register_cpu() returns -EPROBE_DEFER if the particular architecture is not yet ready to answer the question of whether a particular CPU maybe used. This occurs for ARM64 + ACPI in 2 cases. 1) At the initial callsite early in boot, before the AML interpreter is available and so the code can't query _STA. 2) If _STA is queried but a particular CPU is present but not enabled. Those are the ones we are going to hotplug later. For all other architectures and ARM64 DT boots the this deferred flow is not used. - Make the _make_enabled() and _make_not_enabled() flows more similar to the _make_present() and _make_not_present(). There are still sufficient differences that I don't think it makes sense to combine the code, but ensuring the locking and NUMA handling brings them closer together. Note than an additional series will address the question of onlining and offlining the NUMA node as for now it will always be present (that series is not necessary for initial merge of this feature). Dropped RFC because I think this is getting close to ready for merging and now we are interested in normal review rather than calling out significant remaining questions. Updated version of James' original introduction. 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 into an arch specific call made from 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, though for now only arm64 will have deferred the cpu_register() calls. 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. 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 virt-cpuhp-armv8/rfc-v2 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 James Morse (11): ACPI: processor: Register deferred CPUs from acpi_processor_get_info() 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 arm64: acpi: Move get_cpu_for_acpi_id() to a header 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 (un)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 (1): arm64: psci: Ignore DENIED CPUs Jonathan Cameron (5): cpu: Do not warn on arch_register_cpu() returning -EPROBE_DEFER ACPI: processor: Set the ACPI_COMPANION for the struct cpu instance ACPI: utils: Add an acpi_sta_enabled() helper and use it in acpi_processor_make_present() ACPI: scan: Add parameter to allow defering some actions in acpi_scan_check_and_detach. arm64: arch_register_cpu() variant to allow checking of ACPI _STA Russell King (1): ACPI: convert acpi_processor_post_eject() to use IS_ENABLED() .../ABI/testing/sysfs-devices-system-cpu | 6 + Documentation/arch/arm64/cpu-hotplug.rst | 79 ++++++++++++ Documentation/arch/arm64/index.rst | 1 + arch/arm64/include/asm/acpi.h | 11 ++ arch/arm64/kernel/acpi_numa.c | 11 -- arch/arm64/kernel/psci.c | 2 +- arch/arm64/kernel/smp.c | 23 +++- drivers/acpi/acpi_processor.c | 112 +++++++++++++++--- drivers/acpi/scan.c | 57 +++++++-- drivers/acpi/utils.c | 21 ++++ drivers/base/cpu.c | 12 +- drivers/irqchip/irq-gic-v3.c | 32 +++-- include/acpi/acpi_bus.h | 2 + include/linux/acpi.h | 5 +- include/linux/cpumask.h | 25 ++++ kernel/cpu.c | 3 + 16 files changed, 346 insertions(+), 56 deletions(-) create mode 100644 Documentation/arch/arm64/cpu-hotplug.rst -- 2.39.2
