Re: [PATCH V1 0/4] Arch agnostic ACPI changes to support vCPU Hotplug (on Archs like ARM)

[Gustavo Romero <gustavo.romero@linaro.org>]

Hi Miguel,

On 10/15/24 15:41, Miguel Luis wrote:
> 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:

Do you mind sharing what cpus you used for your tests?

Did you use tcg or kvm for the tests?

Thanks!


Cheers,
Gustavo
  
> Tested-by: Miguel Luis <miguel.luis@oracle.com>
> 
> Thanks
> Miguel
> 
>> On 14 Oct 2024, at 19:22, Salil Mehta <salil.mehta@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 <imammedo@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.mehta@huawei.com/T/#mf32be203baa568a871dc625b732f666a4c4f1e68
>> [5] Architecture agnostic ACPI VMSD state migration (Discussion)
>>     Link: https://lore.kernel.org/qemu-devel/20240715155436.577d34c5@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
>>
>