linux-kernel - Re: [PATCH RFC v3 03/21] ACPI: processor: Register CPUs that are online, but not described in the DSDT

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Date: Mon, 18 Dec 2023 21:22:03 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Russell King <rmk+kernel@...linux.org.uk>
Cc: linux-pm@...r.kernel.org, loongarch@...ts.linux.dev, 
	linux-acpi@...r.kernel.org, linux-arch@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
	linux-riscv@...ts.infradead.org, kvmarm@...ts.linux.dev, x86@...nel.org, 
	acpica-devel@...ts.linuxfoundation.org, linux-csky@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-ia64@...r.kernel.org, 
	linux-parisc@...r.kernel.org, Salil Mehta <salil.mehta@...wei.com>, 
	Jean-Philippe Brucker <jean-philippe@...aro.org>, jianyong.wu@....com, justin.he@....com, 
	James Morse <james.morse@....com>
Subject: Re: [PATCH RFC v3 03/21] ACPI: processor: Register CPUs that are
 online, but not described in the DSDT

On Wed, Dec 13, 2023 at 1:49 PM Russell King <rmk+kernel@...linux.org.uk> wrote:
>
> From: James Morse <james.morse@....com>
>
> 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.

So why is this a kernel problem?

> Signed-off-by: James Morse <james.morse@....com>
> Reviewed-by: Jonathan Cameron <Jonathan.Cameron@...wei.com>
> Reviewed-by: Gavin Shan <gshan@...hat.com>
> Tested-by: Miguel Luis <miguel.luis@...cle.com>
> Tested-by: Vishnu Pajjuri <vishnu@...amperecomputing.com>
> Tested-by: Jianyong Wu <jianyong.wu@....com>
> Signed-off-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>
> ---
>  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 6a542e0ce396..0511f2bc10bc 100644
> --- a/drivers/acpi/acpi_processor.c
> +++ b/drivers/acpi/acpi_processor.c
> @@ -791,6 +791,25 @@ 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);

Which part of this code is related to ACPI?

> +               }
> +       }
> +
> +       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.
> --