Re: [PATCH RFC V3 13/29] arm/virt: Make ARM vCPU *present* status ACPI *persistent*

[Nicholas Piggin]

On Fri Jun 14, 2024 at 9:36 AM AEST, Salil Mehta wrote:
> ARM arch does not allow CPUs presence to be changed [1] after kernel has 
> booted.
> Hence, firmware/ACPI/Qemu must ensure persistent view of the vCPUs to the 
> Guest
> kernel even when they are not present in the QoM i.e. are unplugged or are
> yet-to-be-plugged

Do you need arch-independent state for this? If ARM always requires
it then can it be implemented between arm and acpi interface?

If not, then perhaps could it be done in the patch that introduces
all the other state?

> References:
> [1] Check comment 5 in the bugzilla entry
>    Link: https://bugzilla.tianocore.org/show_bug.cgi?id=4481#c5

If I understand correctly (and I don't know ACPI, so it's likely
I don't), that is and update to ACPI spec to say some bit in ACPI
table must remain set regardless of CPU hotplug state.

Reference links are good, I think it would be nice to add a small
summary in the changelog too.

Thanks,
Nick

>
> Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
> ---
>  cpu-common.c          |  6 ++++++
>  hw/arm/virt.c         |  7 +++++++
>  include/hw/core/cpu.h | 21 +++++++++++++++++++++
>  3 files changed, 34 insertions(+)
>
> diff --git a/cpu-common.c b/cpu-common.c
> index 49d2a50835..e4b4dee99a 100644
> --- a/cpu-common.c
> +++ b/cpu-common.c
> @@ -128,6 +128,12 @@ bool qemu_enabled_cpu(CPUState *cpu)
>      return cpu && !cpu->disabled;
>  }
>  
> +bool qemu_persistent_cpu(CPUState *cpu)
> +{
> +    /* cpu state can be faked to the guest via acpi */
> +    return cpu && cpu->acpi_persistent;
> +}
> +
>  uint64_t qemu_get_cpu_archid(int cpu_index)
>  {
>      MachineState *ms = MACHINE(qdev_get_machine());
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 5f98162587..9d33f30a6a 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -3016,6 +3016,13 @@ static void virt_cpu_pre_plug(HotplugHandler 
> *hotplug_dev, DeviceState *dev,
>          return;
>      }
>      virt_cpu_set_properties(OBJECT(cs), cpu_slot, errp);
> +
> +    /*
> +     * To give persistent presence view of vCPUs to the guest, ACPI might 
> need
> +     * to fake the presence of the vCPUs to the guest but keep them disabled.
> +     * This shall be used during the init of ACPI Hotplug state and 
> hot-unplug
> +     */
> +     cs->acpi_persistent = true;
>  }
>  
>  static void virt_cpu_plug(HotplugHandler *hotplug_dev, DeviceState *dev,
> diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
> index 62e68611c0..e13e542177 100644
> --- a/include/hw/core/cpu.h
> +++ b/include/hw/core/cpu.h
> @@ -540,6 +540,14 @@ struct CPUState {
>       * every CPUState is enabled across all architectures.
>       */
>      bool disabled;
> +    /*
> +     * On certain architectures, to provide a persistent view of the 
> 'presence'
> +     * of vCPUs to the guest, ACPI might need to fake the 'presence' of the
> +     * vCPUs but keep them ACPI-disabled for the guest. This is achieved by
> +     * returning `_STA.PRES=True` and `_STA.Ena=False` for the unplugged 
> vCPUs
> +     * in QEMU QoM.
> +     */
> +    bool acpi_persistent;
>  
>      /* TODO Move common fields from CPUArchState here. */
>      int cpu_index;
> @@ -959,6 +967,19 @@ bool qemu_present_cpu(CPUState *cpu);
>   */
>  bool qemu_enabled_cpu(CPUState *cpu);
>  
> +/**
> + * qemu_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.
> + *
> + * Returns: True if it is ACPI 'persistent' CPU
> + *
> + */
> +bool qemu_persistent_cpu(CPUState *cpu);
> +
>  /**
>   * qemu_get_cpu_archid:
>   * @cpu_index: possible vCPU for which arch-id needs to be retreived