[PATCHv3 13/14] x86/acpi: Do not attempt to bring up secondary CPUs in kexec case

[Kirill A. Shutemov]

ACPI MADT doesn't allow to offline CPU after it got woke up. It limits
kexec: the second kernel won't be able to use more than 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.

Booting the second kernel with signle CPU is enough to cover the most
common case for kexec -- kdump.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>
---
 arch/x86/kernel/acpi/madt_wakeup.c | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/arch/x86/kernel/acpi/madt_wakeup.c b/arch/x86/kernel/acpi/madt_wakeup.c
index 386adbb03094..5d92d12f1042 100644
--- a/arch/x86/kernel/acpi/madt_wakeup.c
+++ b/arch/x86/kernel/acpi/madt_wakeup.c
@@ -13,6 +13,11 @@ static struct acpi_madt_multiproc_wakeup_mailbox *acpi_mp_wake_mailbox __ro_afte
 
 static int acpi_wakeup_cpu(u32 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 +83,23 @@ int __init acpi_parse_mp_wake(union acpi_subtable_headers *header,
 
 	cpu_hotplug_disable_offlining();
 
+	/*
+	 * ACPI MADT doesn't allow to offline CPU after it got woke up.
+	 * It limits kexec: the second kernel won't be able to use more than
+	 * 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.
+	 */
+	mp_wake->mailbox_address = 0;
+
 	apic_update_callback(wakeup_secondary_cpu_64, acpi_wakeup_cpu);
 
 	return 0;
-- 
2.41.0