On Thu, Apr 18, 2024 at 3:54 PM Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx> wrote: > > Whilst it is a bit quick after v6, a couple of critical issues > were pointed out by Russell, Salil and Rafael + one build issue > had been missed, so it seems sensible to make sure those conducting > testing or further review have access to a fixed version. > > v7: > - Fix misplaced config guard that broke bisection. > - Greatly simplify the condition on which we call > acpi_processor_hotadd_init(). > - Improve teardown ordering. Thank you for the update! >From a quick look, patches [01-08/16] appear to be good now, but I'll do a more detailed review on the following days. > Fundamental change v6+: At the level of common ACPI infrastructure, use > the existing hotplug path for arm64 even though what needs to be > done at the architecture specific level is quite different. > > An explicit check in arch_register_cpu() for arm64 prevents > this code doing anything if Physical CPU Hotplug is signalled. > > This should resolve any concerns about treating virtual CPU > hotplug as if it were physical and potential unwanted side effects > if physical CPU hotplug is added to the ARM architecture in the > future. > > v6: Thanks to Rafael for extensive help with the approach + reviews. > Specific changes: > - Do not differentiate wrt code flow between traditional CPU HP > and the new ARM flow. The conditions on performing hotplug actions > do need to be adjusted though to incorporate the slightly different > state transition > Added PRESENT + !ENABLED -> PRESENT + ENABLED > to existing !PRESENT + !ENABLED -> PRESENT + ENABLED > - Enable ACPI_HOTPLUG_CPU on arm64 and drop the earlier patches that > took various code out of the protection of that. Now the paths > - New patch to drop unnecessary _STA check in hotplug code. This > code cannot be entered unless ENABLED + PRESENT are set. > - New patch to unify the flow of already onlined (at time of driver > load) and hotplugged CPUs in acpi/processor_driver.c. > This change is necessary because we can't easily distinguish the > 2 cases of deferred vs hotplug calls of register_cpu() on arm64. > It is also a nice simplification. > - Use flags rather than a structure for the extra parameter to > acpi_scan_check_and_detach() - Thank to Shameer for offline feedback. > > 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 and the > kernel beyond arm64 architecture specific code, 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 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 (7): > ACPI: processor: Register deferred CPUs from acpi_processor_get_info() > ACPI: Add post_eject to struct acpi_scan_handler for cpu hotplug > 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 > 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 (8): > ACPI: processor: Simplify initial onlining to use same path for cold > and hotplug > cpu: Do not warn on arch_register_cpu() returning -EPROBE_DEFER > ACPI: processor: Drop duplicated check on _STA (enabled + present) > ACPI: processor: Move checks and availability of acpi_processor > earlier > ACPI: processor: Add acpi_get_processor_handle() helper > ACPI: scan: switch to flags for acpi_scan_check_and_detach() > arm64: arch_register_cpu() variant to check if an ACPI handle is now > available. > arm64: Kconfig: Enable hotplug CPU on arm64 if ACPI_PROCESSOR is > enabled. > > .../ABI/testing/sysfs-devices-system-cpu | 6 + > Documentation/arch/arm64/cpu-hotplug.rst | 79 ++++++++++++ > Documentation/arch/arm64/index.rst | 1 + > arch/arm64/Kconfig | 1 + > arch/arm64/include/asm/acpi.h | 11 ++ > arch/arm64/kernel/acpi.c | 16 +++ > arch/arm64/kernel/acpi_numa.c | 11 -- > arch/arm64/kernel/psci.c | 2 +- > arch/arm64/kernel/smp.c | 56 ++++++++- > drivers/acpi/acpi_processor.c | 113 ++++++++++-------- > drivers/acpi/processor_driver.c | 44 ++----- > drivers/acpi/scan.c | 47 ++++++-- > drivers/base/cpu.c | 12 +- > drivers/irqchip/irq-gic-v3.c | 32 +++-- > include/acpi/acpi_bus.h | 1 + > include/acpi/processor.h | 2 +- > include/linux/acpi.h | 10 +- > include/linux/cpumask.h | 25 ++++ > kernel/cpu.c | 3 + > 19 files changed, 357 insertions(+), 115 deletions(-) > create mode 100644 Documentation/arch/arm64/cpu-hotplug.rst > > -- > 2.39.2 > >
