[PATCH v1] x86/apic: Fix kernel panic when "intremap=off" and "x2apic_phys" are set

[Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>]

x2APIC mode requires "Interrupt Remapping" to be enabled and the
"physical x2apic" driver can be used only when x2APIC mode is enabled.
However when "intremap=off" and "x2apic_phys" kernel command line
parameters are passed, "physical x2apic" driver is being used even when
x2APIC mode is disabled ("intremap=off" disables x2APIC mode).
This results in the below kernel panic:

  unchecked MSR access error: RDMSR from 0x80f at rIP: 0xffffffff87eab4ec
  (native_read_msr+0xc/0x40)
  Call Trace:
  <TASK>
  native_apic_msr_read+0x1f/0x30
  setup_local_APIC+0x4e/0x380
  ? zen_untrain_ret+0x1/0x1
  ? enable_IR_x2apic+0xe8/0x250
  apic_intr_mode_init+0x4c/0x120
  x86_late_time_init+0x28/0x40
  start_kernel+0x626/0xa80
  x86_64_start_reservations+0x1c/0x30
  x86_64_start_kernel+0xbf/0x110
  secondary_startup_64_no_verify+0x10b/0x10b
  </TASK>

This is due to an incorrect conditional check in x2apic_phys_probe().
Fix it here by returning 0 when "x2apic_mode" is not set in
x2apic_phys_probe(). This now prevents default_setup_apic_routing() from
selecting "physical x2apic" driver.

Fixes: 9ebd680bd029 ("x86, apic: Use probe routines to simplify apic selection")
Reviewed-by: Kishon Vijay Abraham I <kvijayab@amd.com>
Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Signed-off-by: Dheeraj Kumar Srivastava <dheerajkumar.srivastava@amd.com>
---
 arch/x86/kernel/apic/x2apic_phys.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/apic/x2apic_phys.c b/arch/x86/kernel/apic/x2apic_phys.c
index 6bde05a86b4e..896bc41cb2ba 100644
--- a/arch/x86/kernel/apic/x2apic_phys.c
+++ b/arch/x86/kernel/apic/x2apic_phys.c
@@ -97,7 +97,10 @@ 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;
-- 
2.25.1