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

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

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@linux.intel.com>
---
 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.
+	 *
+	 * 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;
-- 
2.41.0