[v8,00/16] ACPI/arm64: add support for virtual cpu hotplug

Message

Jonathan Cameron April 26, 2024, 1:51 p.m. UTC

Thanks to Rafael, Marc, Hanjun and Gavin for reviews of v8 and tags.

v8: Details and more minor stuff in individual patch descriptions
  - Rewrite the handling of not enabled redistributors in the gicv3 driver
    to keep impacts local. Note this needs a dance with a new cpuhp
    callback that runs before CPU bring up reaches the section that
    cannot fail. This avoids clearing bits in cpu_present_mask and
    cpu_possible_mask making the solution much less fragile.
  - Do not carry on for that CPU if acpi_processor_set_per_cpu() detects
    a problem, but continue to leave enough in place to perform buggy bios
    checks.
  - Protect against a buggy BIOS that has apparently enabled CPUs in DSDT
    that are not in MADT. The check makes sense anyway as on arm64 we have
    not way to 'generate' an ID if one isn't present at acpi_map_cpu().

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                      |  22 ++++
 arch/arm64/kernel/acpi_numa.c                 |  11 --
 arch/arm64/kernel/psci.c                      |   2 +-
 arch/arm64/kernel/smp.c                       |  56 ++++++++-
 drivers/acpi/acpi_processor.c                 | 115 +++++++++++-------
 drivers/acpi/processor_driver.c               |  44 ++-----
 drivers/acpi/scan.c                           |  47 +++++--
 drivers/base/cpu.c                            |  12 +-
 drivers/irqchip/irq-gic-v3.c                  |  49 ++++++--
 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, 383 insertions(+), 114 deletions(-)
 create mode 100644 Documentation/arch/arm64/cpu-hotplug.rst