Re: [PATCH 12/13] x86/acpi: Do not attempt to bring up secondary CPUs in kexec case

[Huang, Kai]

On Thu, 2023-10-05 at 16:14 +0300, Kirill A. Shutemov wrote:
> ACPI MADT doesn't allow to offline CPU after it got woke up. It limits
> kexec: target kernel won't be able to use more than one CPU.
> 
> Zero out mailbox address in the ACPI MADT wakeup structure to indicate
> that the mailbox is not usable.
> 
> This is Linux-specific protocol and not reflected in ACPI spec.
> 
> Booting the target kernel with signle CPU is enough to cover the most
> common case for kexec -- kdump.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/acpi/madt_wakeup.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/arch/x86/kernel/acpi/madt_wakeup.c b/arch/x86/kernel/acpi/madt_wakeup.c
> index 15bdf10b1393..4e92d1d4a5fa 100644
> --- a/arch/x86/kernel/acpi/madt_wakeup.c
> +++ b/arch/x86/kernel/acpi/madt_wakeup.c
> @@ -9,6 +9,11 @@ static struct acpi_madt_multiproc_wakeup_mailbox *acpi_mp_wake_mailbox;
>  
>  static int acpi_wakeup_cpu(int apicid, unsigned long start_ip)
>  {
> +	if (!acpi_mp_wake_mailbox_paddr) {
> +		pr_warn_once("No MADT mailbox: cannot bringup secondary CPUs. Booting with kexec?\n");
> +		return -EOPNOTSUPP;
> +	}
> +
>  	/*
>  	 * Remap mailbox memory only for the first call to acpi_wakeup_cpu().
>  	 *
> @@ -78,6 +83,18 @@ int __init acpi_parse_mp_wake(union acpi_subtable_headers *header,
>  	/* Disable CPU onlining/offlining */
>  	cpu_hotplug_not_supported();
>  
> +	/*
> +	 * ACPI MADT doesn't allow to offline CPU after it got woke up.
> +	 * It limits kexec: target kernel won't be able to use more than
> +	 * one CPU.
> +	 *
> +	 * Zero out mailbox address in the ACPI MADT wakeup structure to
> +	 * indicate that the mailbox is not usable.

Nit:

It is better to explicitly say that this will only impact the second kernel
because the current kernel has already detected the  mailbox address?

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