Re: [RFC PATCH v2 16/35] ACPI: processor: Register CPUs that are online, but not described in the DSDT

[Jonathan Cameron]

On Wed, 13 Sep 2023 16:38:04 +0000
James Morse <james.morse@xxxxxxx> wrote:

> ACPI has two descriptions of CPUs, one in the MADT/APIC table, the other
> in the DSDT. Both are required. (ACPI 6.5's 8.4 "Declaring Processors"
> says "Each processor in the system must be declared in the ACPI
> namespace"). Having two descriptions allows firmware authors to get
> this wrong.
> 
> If CPUs are described in the MADT/APIC, they will be brought online
> early during boot. Once the register_cpu() calls are moved to ACPI,
> they will be based on the DSDT description of the CPUs. When CPUs are
> missing from the DSDT description, they will end up online, but not
> registered.
> 
> Add a helper that runs after acpi_init() has completed to register
> CPUs that are online, but weren't found in the DSDT. Any CPU that
> is registered by this code triggers a firmware-bug warning and kernel
> taint.
> 
> Qemu TCG only describes the first CPU in the DSDT, unless cpu-hotplug
> is configured.

We should fix that as who likes warnings and taint :)
I dread to think how common this will turn out to be.

> 
> Signed-off-by: James Morse <james.morse@xxxxxxx>
LGTM
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>

> ---
>  drivers/acpi/acpi_processor.c | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
> 
> diff --git a/drivers/acpi/acpi_processor.c b/drivers/acpi/acpi_processor.c
> index b4bde78121bb..a01e315aa16a 100644
> --- a/drivers/acpi/acpi_processor.c
> +++ b/drivers/acpi/acpi_processor.c
> @@ -790,6 +790,25 @@ void __init acpi_processor_init(void)
>  	acpi_pcc_cpufreq_init();
>  }
>  
> +static int __init acpi_processor_register_missing_cpus(void)
> +{
> +	int cpu;
> +
> +	if (acpi_disabled)
> +		return 0;
> +
> +	for_each_online_cpu(cpu) {
> +		if (!get_cpu_device(cpu)) {
> +			pr_err_once(FW_BUG "CPU %u has no ACPI namespace description!\n", cpu);
> +			add_taint(TAINT_FIRMWARE_WORKAROUND, LOCKDEP_STILL_OK);
> +			arch_register_cpu(cpu);
> +		}
> +	}
> +
> +	return 0;
> +}
> +subsys_initcall_sync(acpi_processor_register_missing_cpus);
> +
>  #ifdef CONFIG_ACPI_PROCESSOR_CSTATE
>  /**
>   * acpi_processor_claim_cst_control - Request _CST control from the platform.