Re: [RFC PATCH 00/32] ACPI/arm64: add support for virtual cpuhotplug

From: Shaoqin Huang <shahuang@redhat.com>
To: James Morse <james.morse@arm.com>,
	linux-pm@vger.kernel.org, loongarch@lists.linux.dev,
	kvmarm@lists.linux.dev, kvm@vger.kernel.org,
	linux-acpi@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-ia64@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, x86@kernel.org
Cc: Marc Zyngier <maz@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Lorenzo Pieralisi <lpieralisi@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Borislav Petkov <bp@alien8.de>, H Peter Anvin <hpa@zytor.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Huacai Chen <chenhuacai@kernel.org>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Oliver Upton <oliver.upton@linux.dev>,
	Len Brown <lenb@kernel.org>, Rafael Wysocki <rafael@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Salil Mehta <salil.mehta@huawei.com>,
	Russell King <linux@armlinux.org.uk>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>
Subject: Re: [RFC PATCH 00/32] ACPI/arm64: add support for virtual cpuhotplug
Date: Wed, 29 Mar 2023 13:52:07 +0800	[thread overview]
Message-ID: <e17627fb-283e-dd42-94c1-f89dea167577@redhat.com> (raw)
In-Reply-To: <20230203135043.409192-1-james.morse@arm.com>

Hi James,

On 2/3/23 21:50, James Morse wrote:
> Hello!
> 
> 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.
> 
> 
> 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

I applied this patch series on v6.2-rc3 and using the QEMU cloned from 
the salil-mehta/qemu.git repo. But when I try to run the QEMU, it shows:

$ qemu-system-aarch64: -accel kvm: Failed to enable 
KVM_CAP_ARM_PSCI_TO_USER cap.

Here is the command I use:

$ qemu-system-aarch64
-machine virt
-bios /usr/share/qemu-efi-aarch64/QEMU_EFI.fd
-accel kvm
-m 4096
-smp cpus=1,maxcpus=3,cores=3,threads=1,sockets=1
-cpu host
-qmp unix:./src.socket,server,nowait
-hda ./XXX.qcow2
-serial unix:./src.serial,server,nowait
-monitor stdio

It seems something related to your notice: 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.

But I'm not actually understand what should I fix, since I haven't 
review the patch series. Could you give me some more information? Maybe 
I'm doing something wrong.

Thanks,

> 
> 
> 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
> 

-- 
Shaoqin

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