x86/apic: Fix kernel panic when booting with intremap=off and x2apic_…

…phys

[ Upstream commit 85d38d5810e285d5aec7fb5283107d1da70c12a9 ]

When booting with "intremap=off" and "x2apic_phys" on the kernel command
line, the physical x2APIC driver ends up being used even when x2APIC
mode is disabled ("intremap=off" disables x2APIC mode). This happens
because the first compound condition check in x2apic_phys_probe() is
false due to x2apic_mode == 0 and so the following one returns true
after default_acpi_madt_oem_check() having already selected the physical
x2APIC driver.

This results in the following panic:

   kernel BUG at arch/x86/kernel/apic/io_apic.c:2409!
   invalid opcode: 0000 [PeterCxy#1] PREEMPT SMP NOPTI
   CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.4.0-rc2-ver4.1rc2 OnePlusOSS#2
   Hardware name: Dell Inc. PowerEdge R6515/07PXPY, BIOS 2.3.6 07/06/2021
   RIP: 0010:setup_IO_APIC+0x9c/0xaf0
   Call Trace:
    <TASK>
    ? native_read_msr
    apic_intr_mode_init
    x86_late_time_init
    start_kernel
    x86_64_start_reservations
    x86_64_start_kernel
    secondary_startup_64_no_verify
    </TASK>

which is:

setup_IO_APIC:
  apic_printk(APIC_VERBOSE, "ENABLING IO-APIC IRQs\n");
  for_each_ioapic(ioapic)
  	BUG_ON(mp_irqdomain_create(ioapic));

Return 0 to denote that x2APIC has not been enabled when probing the
physical x2APIC driver.

  [ bp: Massage commit message heavily. ]

Fixes: 9ebd680 ("x86, apic: Use probe routines to simplify apic selection")
Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@gmail.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20230616212236.1389-1-dheerajkumar.srivastava@amd.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

arch/x86/kernel/apic/x2apic_phys.c

@@ -95,7 +95,10 @@ static void init_x2apic_ldr(void)
 
 static int x2apic_phys_probe(void)
 {
-	if (x2apic_mode && (x2apic_phys || x2apic_fadt_phys()))
+	if (!x2apic_mode)
+		return 0;
+
+	if (x2apic_phys || x2apic_fadt_phys())
 		return 1;
 
 	return apic == &apic_x2apic_phys;