[PATCH v6 00/16] ACPI/arm64: add support for virtual cpu hotplug

Jonathan Cameron
2024-04-17T13:19:14Z

Fundamental change: 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 | 112 +++++++++++-------
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, 362 insertions(+), 109 deletions(-)
create mode 100644 Documentation/arch/arm64/cpu-hotplug.rst

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