[PATCHv4 13/14] x86/acpi: Do not attempt to bring up secondary CPUs in kexec case

[Thomas Gleixner]

On Tue, Dec 05 2023 at 03:45, Kirill A. Shutemov wrote:
> ACPI MADT doesn't allow to offline CPU after it got woke up. It limits

to offline a CPU after it was onlined. This limits kexec: ...

> kexec: the second kernel won't be able to use more than one CPU.

... one CPU, which is enough to cover the kdump case.


> Now acpi_mp_wake_mailbox_paddr already has the mailbox address.
> The acpi_wakeup_cpu() will use it to bring up secondary cpus.
>
> Zero out mailbox address in the ACPI MADT wakeup structure to indicate
> that the mailbox is not usable.  This prevents the kexec()-ed kernel
> from reading a vaild mailbox, which in turn makes the kexec()-ed kernel
> only be able to use the boot CPU.
>
> This is Linux-specific protocol and not reflected in ACPI spec.
>
> Booting the second kernel with signle CPU is enough to cover the most
> common case for kexec -- kdump.

This is confusing at best and I doubt that kdump is the most common case
for every one.

  To prevent a kexec kernel from onlining secondary CPUs invalidate the
  mailbox address in the ACPI MADT wakeup structure which prevents a
  kexec kernel to use it.

  This is safe as the booting kernel has the mailbox address cached
  already and acpi_wakeup_cpu() uses the cached value to bring up the
  secondary CPUs.

  Note: This is a Linux specific convention and not covered by the
        ACPI specification.

Hmm?

> +	/*
> +	 * ACPI MADT doesn't allow to offline CPU after it got woke up.

to offline a CPU after it was onlined.

> +	 * It limits kexec: the second kernel won't be able to use more than

           This limits kexec: ...

> +	 * one CPU.
> +	 *
> +	 * Now acpi_mp_wake_mailbox_paddr already has the mailbox address.
> +	 * The acpi_wakeup_cpu() will use it to bring up secondary cpus.
> +	 *
> +	 * Zero out mailbox address in the ACPI MADT wakeup structure to
> +	 * indicate that the mailbox is not usable.  This prevents the
> +	 * kexec()-ed kernel from reading a vaild mailbox, which in turn
> +	 * makes the kexec()-ed kernel only be able to use the boot CPU.
> +	 *
> +	 * This is Linux-specific protocol and not reflected in ACPI spec.

See changelog comment...

> +	 */
> +	mp_wake->mailbox_address = 0;
> +
>  	apic_update_callback(wakeup_secondary_cpu_64, acpi_wakeup_cpu);
>  
>  	return 0;