Re: [RFC PATCH v3 00/39] ACPI/arm64: add support for virtual cpuhotplug

Miguel Luis
2023-10-30T16:42:30Z

Hi Russell,

> On 24 Oct 2023, at 15:15, Russell King (Oracle) <linux@armlinux.org.uk> wrote:
>
> Hi,
>
> I'm posting James' patch set updated with most of the review comments
> from his RFC v2 series back in September. Individual patches have a
> changelog attached at the bottom of the commit message. Those which
> I have finished updating have my S-o-b on them, those which still have
> outstanding review comments from RFC v2 do not. In some of these cases
> I've asked questions and am waiting for responses.
>
> I'm posting this as RFC v3 because there's still some unaddressed
> comments and it's clearly not ready for merging. Even if it was ready
> to be merged, it is too late in this development cycle to be taking
> this change in, so there would be little point posting it non-RFC.
> Also James stated that he's waiting for confirmation from the
> Kubernetes/Kata folk - I have no idea what the status is there.
>
> I will be sending each patch individually to a wider audience
> appropriate for that patch - apologies to those missing out on this
> cover message. I have added more mailing lists to the series with the
> exception of the acpica list in a hope of this cover message also
> reaching those folk.
>
> The changes that aren't included are:
>
> 1. Updates for my patch that was merged via Thomas (thanks!):
> c4dd854f740c cpu-hotplug: Provide prototypes for arch CPU registration
> rather than having this change spread through James' patches.
>
> 2. New patch - simplification of PA-RISC's smp_prepare_boot_cpu()
>
> 3. Moved "ACPI: Use the acpi_device_is_present() helper in more places"
> and "ACPI: Rename acpi_scan_device_not_present() to be about
> enumeration" to the beginning of the series - these two patches are
> already queued up for merging into 6.7.
>
> 4. Moved "arm64, irqchip/gic-v3, ACPI: Move MADT GICC enabled check into
> a helper" to the beginning of the series, which has been submitted,
> but as yet the fate of that posting isn't known.
>
> The first four patches in this series are provided for completness only.
>
> There is an additional patch in James' git tree that isn't in the set
> of patches that James posted: "ACPI: processor: Only call
> arch_unregister_cpu() if HOTPLUG_CPU is selected" which looks to me to
> be a workaround for arch_unregister_cpu() being under the ifdef. I've
> commented on this on the RFC v2 posting making a suggestion, but as yet
> haven't had any response.
>
> I've included almost all of James' original covering body below the
> diffstat.
>
> The reason that I'm doing this is to help move this code forward so
> hopefully it can be merged - which is why I have been keen to dig out
> from James' patches anything that can be merged and submit it
> separately, since this is a feature for which some users have a
> definite need for.
>
> Please note that I haven't tested this beyond building for aarch64 at
> the present time.
>
> The series can be found at:
>
> git://git.armlinux.org.uk/~rmk/linux-arm.git aarch64/hotplug-vcpu/v6.6-rc7
>
> 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/include/asm/cpu.h | 1 -
> 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/ia64/Kconfig | 3 +
> arch/ia64/include/asm/acpi.h | 2 +-
> arch/ia64/include/asm/cpu.h | 6 --
> arch/ia64/kernel/acpi.c | 6 +-
> arch/ia64/kernel/setup.c | 2 +-
> arch/ia64/kernel/topology.c | 35 +------
> arch/loongarch/Kconfig | 2 +
> arch/loongarch/configs/loongson3_defconfig | 2 +-
> arch/loongarch/kernel/acpi.c | 4 +-
> arch/loongarch/kernel/topology.c | 38 +-------
> arch/parisc/kernel/smp.c | 8 +-
> arch/riscv/Kconfig | 1 +
> arch/riscv/kernel/setup.c | 19 +---
> arch/x86/Kconfig | 3 +
> arch/x86/include/asm/cpu.h | 4 -
> arch/x86/kernel/acpi/boot.c | 4 +-
> arch/x86/kernel/cpu/intel_epb.c | 2 +-
> arch/x86/kernel/topology.c | 27 +-----
> drivers/acpi/Kconfig | 14 ++-
> drivers/acpi/acpi_processor.c | 151 +++++++++++++++++++++++------
> drivers/acpi/bus.c | 16 +++
> drivers/acpi/device_pm.c | 2 +-
> drivers/acpi/device_sysfs.c | 2 +-
> drivers/acpi/internal.h | 1 -
> drivers/acpi/processor_core.c | 2 +-
> drivers/acpi/property.c | 2 +-
> drivers/acpi/scan.c | 148 ++++++++++++++++++----------
> drivers/base/arch_topology.c | 38 +++++---
> drivers/base/cpu.c | 44 +++++++--
> drivers/base/init.c | 2 +-
> drivers/base/node.c | 7 --
> 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/linux/acpi.h | 13 ++-
> include/linux/cpu.h | 4 +
> include/linux/cpumask.h | 25 +++++
> kernel/cpu.c | 3 +
> 48 files changed, 516 insertions(+), 292 deletions(-)
>
>
> On Wed, Sep 13, 2023 at 04:37:48PM +0000, James Morse wrote:
>> Hello!
>>
>> Changes since RFC-v1:
>> * riscv is new, ia64 is gone
>> * The KVM support is different, and upstream - no need to patch the host.
>>
>> ---
>>
>> 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.
>>
>> I've only build tested Loongarch and riscv. I've removed the ia64 specific
>> patches, but left the changes in other patches to make git-grep review of
>> renames easier.
>>
>> 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-v2-rc6
>>
>> 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
>>
>>
>> Why is this still an RFC? I'm still looking for confirmation from the
>> kubernetes/kata folk that this works for them. Because of this I've culled
>> the CC list...
>>
>>
>> This series is based on v6.6-rc1, and can be retrieved from:
>> https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/ virtual_cpu_hotplug/rfc/v2
>>
>>
>> Thanks,
>>
>> James Morse (34):
>> ACPI: Move ACPI_HOTPLUG_CPU to be disabled on arm64 and riscv
>> 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()
>> drivers: base: Print a warning instead of panic() when register_cpu()
>> fails
>> arm64: setup: Switch over to GENERIC_CPU_DEVICES using
>> arch_register_cpu()
>> x86: intel_epb: Don't rely on link order
>> x86/topology: Switch over to GENERIC_CPU_DEVICES
>> LoongArch: Switch over to GENERIC_CPU_DEVICES
>> riscv: Switch over to GENERIC_CPU_DEVICES
>> arch_topology: Make register_cpu_capacity_sysctl() tolerant to late
>> CPUs
>> ACPI: Use the acpi_device_is_present() helper in more places
>> ACPI: Rename acpi_scan_device_not_present() to be about enumeration
>> ACPI: Only enumerate enabled (or functional) devices
>> 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
>> ACPI: Add _OSC bits to advertise OS support for toggling CPU
>> present/enabled
>> cpumask: Add enabled cpumask for present CPUs that can be brought
>> online
>>
>> Jean-Philippe Brucker (1):
>> arm64: psci: Ignore DENIED CPUs
>

Tested on QEMU, based on Salil's RFC v2 [1], running with KVM.
- boot
- hotplug up to 'maxcpus'
- hotunplug down to the number of boot cpus
- hotplug vcpus and migrate with vcpus offline
- hotplug vcpus and migrate with vcpus online
- hotplug vcpus then unplug vcpus then migrate
- successive live migrations (up until 6)

Feel free to add:
Tested-by: Miguel Luis <miguel.luis@oracle.com>

Thank you
Miguel

[1] https://lore.kernel.org/qemu-devel/20230926100436.28284-1-salil.mehta@huawei.com/

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