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

[Salil Mehta via <qemu-devel@nongnu.org>]

HI Bibo,

>  From: maobibo <maobibo@loongson.cn>
>  Sent: Tuesday, October 15, 2024 4:31 AM
>  To: Salil Mehta <salil.mehta@huawei.com>; qemu-devel@nongnu.org;
>  qemu-arm@nongnu.org; mst@redhat.com
>  
>  With cpu-add/cpu-del command tested on LoongArch system, no migration
>  tested. There is no negative influence with LoongArch cpu hotplug.

Thanks for the confirmation. 

Just curious are guys still using cpu-{add,del} interface for vCPU hotplug?
I thought it was deprecated for device_{add,del}?

https://wiki.qemu.org/Features/CPUHotplug
https://www.qemu.org/docs/master/system/cpu-hotplug.html


Thanks
Salil.

>  
>  Regards
>  Bibo Mao
>  
>  On 2024/10/15 上午3:22, Salil Mehta via 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_Vir
>  > t_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.us
>  > ers.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(-)
>  >