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On most architectures, during vCPU hot-plug and hot-unplug actions, the
firmware or VMM/QEMU can update the OS on vCPU status by toggling the
ACPI method `_STA.Present` bit. However, certain CPU architectures
prohibit [1] modifications to a CPU’s `presence` status after the kernel
has booted.
This limitation [2][3] exists because many per-CPU components, such as
interrupt controllers and various per-CPU features tightly integrated
with CPUs, may not support reconfiguration once the kernel is
initialized. Often, these components cannot be powered down, as they may
belong to an `always-on` power domain. As a result, some architectures
require all CPUs to remain `_STA.Present` after system initialization.
Therefore, it is essential to mirror the exact QOM vCPU status through
ACPI for the Guest kernel. For this, we should determine—via
architecture-specific code[4]—whether vCPUs must always remain present
and whether the associated `AcpiCpuStatus::cpu` object should remain
valid, even following a vCPU hot-unplug operation.
References:
[1] Check comment 5 in the bugzilla entry
Link: https://bugzilla.tianocore.org/show_bug.cgi?id=4481#c5
[2] 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
[3] KVMForum 2020 Presentation: Challenges in Supporting Virtual CPU Hotplug on
SoC Based Systems (like ARM64)
Link: https://kvmforum2020.sched.com/event/eE4m
[4] Example implementation of architecture-specific CPU persistence hook
Link:
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| .. | ||
| accel-cpu.h | ||
| cpu.h | ||
| generic-loader.h | ||
| resetcontainer.h | ||
| split-irq.h | ||
| sysbus-fdt.h | ||
| sysemu-cpu-ops.h | ||
| tcg-cpu-ops.h | ||