On Fri, 3 Feb 2023 13:50:11 +0000
James Morse <james.morse@xxxxxxx> wrote:
...
> 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.
Hi James,
As we discussed on an LOD call a while back, I think that we need some path to
find out if the guest supports vCPU HP or not so that info can be queried by
an orchestrator / libvirt etc. In general the entity responsible for allocating
extra vCPUs may not know what support the VM has for this feature.
There are various ways we could get this information into the VMM.
My immediate thought is to use one of the ACPI interfaces that lets us write
AML that can set an emulated register. A query to the VMM can check if this
register is set.
So options.
_OSI() - Deprecated on ARM64 so lets not use that ;)
_OSC() - Could add a bit to Table 6.13 Platform-Wide Capabilites in ACPI 6.5 spec.
Given x86 has a similar online capable bit perhaps this is the best option
though it is the one that requires a formal code first proposal to ASWG.
_OSC() - Could add a new UUID and put it under a suitable device - maybe all CPUs?
You could definitely argue this feature is an operating system property.
_DSM() - Similar to OSC but always under a device.
Whilst can be used for this I'm not sure it really matches intended usecase.
Assuming everyone agrees this bit of introspection is useful,
Rafael / other ACPI specialists: Any suggestions on how best to do this?
Jonathan
>
> 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.
>
>
> The first patch has already been posted as a fix here:
> https://www.spinics.net/lists/linux-ia64/msg21920.html
> I've only build tested Loongarch and ia64.
>
>
> 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-v1-port29092022.psci.present
>
> You'll need to fix the numbers of KVM_CAP_ARM_HVC_TO_USER and KVM_CAP_ARM_PSCI_TO_USER
> to match your host kernel. 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
>
>
> This series is based on v6.2-rc3, and can be retrieved from:
> https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/ virtual_cpu_hotplug/rfc/v1
>
>
> Thanks,
>
> James Morse (29):
> ia64: Fix build error due to switch case label appearing next to
> declaration
> ACPI: Move ACPI_HOTPLUG_CPU to be enabled per architecture
> 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()
> arm64: setup: Switch over to GENERIC_CPU_DEVICES using
> arch_register_cpu()
> ia64/topology: Switch over to GENERIC_CPU_DEVICES
> x86/topology: Switch over to GENERIC_CPU_DEVICES
> LoongArch: Switch over to GENERIC_CPU_DEVICES
> arch_topology: Make register_cpu_capacity_sysctl() tolerant to late
> CPUs
> 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
> cpumask: Add enabled cpumask for present CPUs that can be brought
> online
>
> Jean-Philippe Brucker (3):
> arm64: psci: Ignore DENIED CPUs
> KVM: arm64: Pass hypercalls to userspace
> KVM: arm64: Pass PSCI calls to userspace
>
> Documentation/arm64/cpu-hotplug.rst | 79 ++++++++++++
> Documentation/arm64/index.rst | 1 +
> Documentation/virt/kvm/api.rst | 31 ++++-
> Documentation/virt/kvm/arm/hypercalls.rst | 1 +
> arch/arm64/Kconfig | 1 +
> arch/arm64/include/asm/acpi.h | 11 ++
> arch/arm64/include/asm/cpu.h | 1 -
> arch/arm64/include/asm/kvm_host.h | 2 +
> 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/arm64/kvm/arm.c | 15 ++-
> arch/arm64/kvm/hypercalls.c | 28 ++++-
> arch/arm64/kvm/psci.c | 13 ++
> arch/ia64/Kconfig | 2 +
> arch/ia64/include/asm/acpi.h | 2 +-
> arch/ia64/include/asm/cpu.h | 11 --
> arch/ia64/kernel/acpi.c | 6 +-
> arch/ia64/kernel/setup.c | 2 +-
> arch/ia64/kernel/sys_ia64.c | 7 +-
> arch/ia64/kernel/topology.c | 35 +-----
> arch/loongarch/Kconfig | 2 +
> arch/loongarch/kernel/topology.c | 31 +----
> arch/x86/Kconfig | 2 +
> arch/x86/include/asm/cpu.h | 6 -
> arch/x86/kernel/acpi/boot.c | 4 +-
> arch/x86/kernel/topology.c | 19 +--
> drivers/acpi/Kconfig | 5 +-
> drivers/acpi/acpi_processor.c | 146 +++++++++++++++++-----
> drivers/acpi/processor_core.c | 2 +-
> drivers/acpi/scan.c | 116 +++++++++++------
> drivers/base/arch_topology.c | 38 ++++--
> drivers/base/cpu.c | 31 ++++-
> drivers/base/init.c | 2 +-
> 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/kvm/arm_hypercalls.h | 1 +
> include/kvm/arm_psci.h | 4 +
> include/linux/acpi.h | 10 +-
> include/linux/cpu.h | 6 +
> include/linux/cpumask.h | 25 ++++
> include/uapi/linux/kvm.h | 2 +
> kernel/cpu.c | 3 +
> 46 files changed, 532 insertions(+), 244 deletions(-)
> create mode 100644 Documentation/arm64/cpu-hotplug.rst
>
