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;
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