x86/acpi: Remove __ro_after_init from acpi_mp_wake_mailbox

On a platform using the "Multiprocessor Wakeup Structure"[1] to startup
secondary CPUs the control processor needs to memremap() the physical
address of the MP Wakeup Structure mailbox to the variable
acpi_mp_wake_mailbox, which holds the virtual address of mailbox.

To wake up the AP the control processor writes the APIC ID of AP, the
wakeup vector and the ACPI_MP_WAKE_COMMAND_WAKEUP command into the mailbox.

Current implementation doesn't consider the case which restricts boot time
CPU bringup to 1 with the kernel parameter "maxcpus=1" and brings other
CPUs online later from user space as it sets acpi_mp_wake_mailbox to
read-only after init.  So when the first AP is tried to brought online
after init, the attempt to update the variable results in a kernel panic.

The memremap() call that initializes the variable cannot be moved into
acpi_parse_mp_wake() because memremap() is not functional at that point in
the boot process. Also as the APs might never be brought up, keep the
memremap() call in acpi_wakeup_cpu() so that the operation only takes place
when needed.

Fixes: 24dd05d ("x86/apic: Mark acpi_mp_wake_* variables as __ro_after_init")
Signed-off-by: Zhiquan Li <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Kirill A. Shutemov <[email protected]>
Link: https://lore.kernel.org/all/[email protected]

Author: zhiquan1-li

arch/x86/kernel/acpi/madt_wakeup.c

@@ -19,7 +19,7 @@
 static u64 acpi_mp_wake_mailbox_paddr __ro_after_init;
 
 /* Virtual address of the Multiprocessor Wakeup Structure mailbox */
-static struct acpi_madt_multiproc_wakeup_mailbox *acpi_mp_wake_mailbox __ro_after_init;
+static struct acpi_madt_multiproc_wakeup_mailbox *acpi_mp_wake_mailbox;
 
 static u64 acpi_mp_pgd __ro_after_init;
 static u64 acpi_mp_reset_vector_paddr __ro_after_init;