[RFC PATCH v3 00/21] ACPI/arm64: add support for virtual cpu hotplug

Russell King (Oracle)
2023-12-13T12:47:44Z

Hi,

This is this remaining patches for ARM64 virtual cpu hotplug, which
follows on from the previous set of 21 patches that GregKH has
recently queued up, and "x86: intel_epb: Don't rely on link order"
which can be found at:

https://lore.kernel.org/r/E1r6SeD-00DCuK-M6@rmk-PC.armlinux.org.uk
https://lore.kernel.org/r/ZVyz/Ve5pPu8AWoA@shell.armlinux.org.uk

The entire series can be found at:

git://git.armlinux.org.uk/~rmk/linux-arm.git aarch64/hotplug-vcpu/head

The original cover message from the entire series is below the
diffstat.

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 | 3 +-
arch/loongarch/Kconfig | 2 +-
arch/loongarch/configs/loongson3_defconfig | 2 +-
arch/loongarch/kernel/acpi.c | 4 +-
arch/x86/Kconfig | 3 +-
arch/x86/kernel/acpi/boot.c | 4 +-
drivers/acpi/Kconfig | 13 ++-
drivers/acpi/acpi_processor.c | 141 ++++++++++++++++++++++++++---
drivers/acpi/bus.c | 16 ++++
drivers/acpi/device_pm.c | 2 +-
drivers/acpi/device_sysfs.c | 2 +-
drivers/acpi/internal.h | 1 -
drivers/acpi/property.c | 2 +-
drivers/acpi/scan.c | 140 ++++++++++++++++++----------
drivers/base/cpu.c | 16 +++-
drivers/irqchip/irq-gic-v3.c | 32 ++++---
include/acpi/acpi_bus.h | 1 +
include/acpi/actbl2.h | 1 +
include/linux/acpi.h | 10 +-
include/linux/cpumask.h | 25 +++++
kernel/cpu.c | 3 +
26 files changed, 421 insertions(+), 106 deletions(-)

On Tue, Oct 24, 2023 at 04:15:28PM +0100, Russell King (Oracle) 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
>
> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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>

--
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