Hi Salil,
I’ve ran the usual tests successfully of hotplug/unplug from the number of
cold-booted cpus up to maxcpus and migration on ARM. Please feel free to add:
Tested-by: Miguel Luis <miguel.l...@oracle.com>
Thanks
Miguel
> On 14 Oct 2024, at 19:22, Salil Mehta <salil.me...@huawei.com> wrote:
>
> Certain CPU architecture specifications [1][2][3] prohibit changes to the CPUs
> *presence* after the kernel has booted. This is because many system
> initializations depend on the exact CPU count at boot time and do not expect
> it
> to change afterward. For example, components like interrupt controllers that
> are
> closely coupled with CPUs, or various per-CPU features, may not support
> configuration changes once the kernel has been initialized.
>
> This requirement poses a challenge for virtualization features like vCPU
> hotplug. To address this, changes to the ACPI AML are necessary to update the
> `_STA.PRES` (presence) and `_STA.ENA` (enabled) bits accordingly during guest
> initialization, as well as when vCPUs are hot-plugged or hot-unplugged. The
> presence of unplugged vCPUs may need to be deliberately *simulated* at the
> ACPI
> level to maintain a *persistent* view of vCPUs for the guest kernel.
>
> This patch set introduces the following features:
>
> 1. ACPI Interface with Explicit PRESENT and ENABLED CPU States: It allows the
> guest kernel to evaluate these states using the `_STA` ACPI method.
>
> 2. Initialization of ACPI CPU States: These states are initialized during
> `machvirt_init` and when vCPUs are hot-(un)plugged. This enables
> hotpluggable
> vCPUs to be exposed to the guest kernel via ACPI.
>
> 3. Support for Migrating ACPI CPU States: The patch set ensures the migration
> of
> the newly introduced `is_{present,enabled}` ACPI CPU states to the
> destination VM.
>
> The approach is flexible enough to accommodate ARM-like architectures that
> intend to implement vCPU hotplug functionality. It is suitable for
> architectures
> facing similar constraints to ARM or those that plan to implement vCPU
> hotplugging independently of hardware support (if available).
>
> This patch set is derived from the ARM-specific vCPU hotplug implementation
> [4]
> and includes migration components adaptable to other architectures, following
> suggestions [5] made by Igor Mammedov <imamm...@redhat.com>.
>
> It can be applied independently, ensuring compatibility with existing hotplug
> support in other architectures. I have tested this patch set in conjunction
> with
> the ARM-specific vCPU hotplug changes (included in the upcoming RFC V5 [6]),
> and
> everything worked as expected. I kindly request maintainers of other
> architectures to provide a "Tested-by" after running their respective
> regression
> tests.
>
> Many thanks!
>
>
> References:
> [1] KVMForum 2023 Presentation: Challenges Revisited in Supporting Virt CPU
> Hotplug on
> architectures that don’t Support CPU Hotplug (like ARM64)
> a. Kernel Link:
> https://kvm-forum.qemu.org/2023/KVM-forum-cpu-hotplug_7OJ1YyJ.pdf
> b. Qemu Link:
> https://kvm-forum.qemu.org/2023/Challenges_Revisited_in_Supporting_Virt_CPU_Hotplug_-__ii0iNb3.pdf
> [2] KVMForum 2020 Presentation: Challenges in Supporting Virtual CPU Hotplug
> on
> SoC Based Systems (like ARM64)
> Link: https://kvmforum2020.sched.com/event/eE4m
> [3] Check comment 5 in the bugzilla entry
> Link: https://bugzilla.tianocore.org/show_bug.cgi?id=4481#c5
> [4] [PATCH RFC V4 00/33] Support of Virtual CPU Hotplug for ARMv8 Arch
> Link:
> https://lore.kernel.org/qemu-devel/20241009031815.250096-1-salil.me...@huawei.com/T/#mf32be203baa568a871dc625b732f666a4c4f1e68
> [5] Architecture agnostic ACPI VMSD state migration (Discussion)
> Link:
> https://lore.kernel.org/qemu-devel/20240715155436.577d3...@imammedo.users.ipa.redhat.com/
> [6] Upcoming RFC V5, Support of Virtual CPU Hotplug for ARMv8 Arch
> Link: https://github.com/salil-mehta/qemu/commits/virt-cpuhp-armv8/rfc-v5
>
> Salil Mehta (4):
> hw/acpi: Initialize ACPI Hotplug CPU Status with Support for vCPU
> `Persistence`
> hw/acpi: Update ACPI CPU Status `is_{present, enabled}` during vCPU
> hot(un)plug
> hw/acpi: Reflect ACPI vCPU {present,enabled} states in ACPI
> _STA.{PRES,ENA} Bits
> hw/acpi: Populate vCPU Hotplug VMSD to migrate `is_{present,enabled}`
> states
>
> cpu-target.c patches.vcpuhp.rfc-v5.arch.agnostic.acpi | 1 +
> hw/acpi/cpu.c | 70 +++++++++++++++++++++++++++++++---
> hw/acpi/generic_event_device.c | 11 ++++++
> include/hw/acpi/cpu.h | 21 ++++++++++
> include/hw/core/cpu.h | 21 ++++++++++
> 5 files changed, 119 insertions(+), 5 deletions(-)
>
> --
> 2.34.1
>
