Re: [PATCH V1 1/4] hw/acpi: Initialize ACPI Hotplug CPU Status with Support for vCPU `Persistence`

[Gavin Shan <gshan@redhat.com>]

On 10/15/24 5:22 AM, Salil Mehta wrote:
> Certain CPU architecture specifications [1][2][3] prohibit changes to CPU
> presence after the kernel has booted. This limitation exists because many system
> initializations rely on the exact CPU count at boot time and do not expect it to
> change later. For example, components like interrupt controllers, which are
> closely tied to CPUs, or various per-CPU features, may not support configuration
> changes once the kernel has been initialized. This presents a challenge for
> virtualization features such as vCPU hotplug.
> 
> To address this issue, introduce an `is_enabled` state in the `AcpiCpuStatus`,
> which reflects whether a vCPU has been hot-plugged or hot-unplugged in QEMU,
> marking it as (un)available in the Guest Kernel. The `is_present` state should
> be set based on the `acpi_persistent` flag. In cases where unplugged vCPUs need
> to be deliberately simulated in the ACPI to maintain a persistent view of vCPUs,
> this flag ensures the guest kernel continues to see those vCPUs.
> 
> Additionally, introduce an `acpi_persistent` property that can be used to
> initialize the ACPI vCPU presence state accordingly. Architectures requiring
> ACPI to expose a persistent view of vCPUs can override its default value. Refer
> to the patch-set implelenting vCPU hotplug support for ARM for more details on
> its usage.
> 
> 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
> 
> Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
> ---
>   cpu-target.c          |  1 +
>   hw/acpi/cpu.c         | 35 ++++++++++++++++++++++++++++++++++-
>   include/hw/acpi/cpu.h | 21 +++++++++++++++++++++
>   include/hw/core/cpu.h | 21 +++++++++++++++++++++
>   4 files changed, 77 insertions(+), 1 deletion(-)
> 

[...]
    
> +/**
> + * acpi_persistent_cpu:
> + * @cpu: The vCPU to check
> + *
> + * Checks if the vCPU state should always be reflected as *present* via ACPI
> + * to the Guest. By default, this is False on all architectures and has to be
> + * explicity set during initialization.
       ^^^^^^^^^
       explicitly

> + *
> + * Returns: True if it is ACPI 'persistent' CPU
> + *
> + */
> +static inline bool acpi_persistent_cpu(CPUState *cpu)
> +{
> +    /*
> +     * returns if 'Presence' of the vCPU is persistent and should be simulated
> +     * via ACPI even after vCPUs have been unplugged in QOM
> +     */
> +    return cpu && cpu->acpi_persistent;
> +}
>   #endif

Thanks,
Gavin