x86/apic: Handle no CONFIG_X86_X2APIC on systems with x2APIC enabled by BIOS

A kernel that was compiled without CONFIG_X86_X2APIC was unable to boot on
platforms that have x2APIC already enabled in the BIOS before starting the
kernel.

The kernel was supposed to panic with an approprite error message in
validate_x2apic() due to the missing X2APIC support.

However, validate_x2apic() was run too late in the boot cycle, and the
kernel tried to initialize the APIC nonetheless. This resulted in an
earlier panic in setup_local_APIC() because the APIC was not registered.

In my experiments, a panic message in setup_local_APIC() was not visible
in the graphical console, which resulted in a hang with no indication
what has gone wrong.

Instead of calling panic(), disable the APIC, which results in a somewhat
working system with the PIC only (and no SMP). This way the user is able to
diagnose the problem more easily.

Disabling X2APIC mode is not an option because it's impossible on systems
with locked x2APIC.

The proper place to disable the APIC in this case is in check_x2apic(),
which is called early from setup_arch(). Doing this in
__apic_intr_mode_select() is too late.

Make check_x2apic() unconditionally available and remove the empty stub.

Reported-by: Paul Menzel <pmenzel@molgen.mpg.de>
Reported-by: Robert Elliott (Servers) <elliott@hpe.com>
Signed-off-by: Mateusz Jończyk <mat.jonczyk@o2.pl>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/lkml/d573ba1c-0dc4-3016-712a-cc23a8a33d42@molgen.mpg.de
Link: https://lore.kernel.org/lkml/20220911084711.13694-3-mat.jonczyk@o2.pl
Link: https://lore.kernel.org/all/20221129215008.7247-1-mat.jonczyk@o2.pl

1 files changed, 8 insertions, 5 deletions

diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c
index c6876d3ea4b1..20d9a604da7c 100644
--- a/arch/x86/kernel/apic/apic.c
+++ b/arch/x86/kernel/apic/apic.c
@@ -1931,16 +1931,19 @@ void __init check_x2apic(void)
 	}
 }
 #else /* CONFIG_X86_X2APIC */
-static int __init validate_x2apic(void)
+void __init check_x2apic(void)
 {
 	if (!apic_is_x2apic_enabled())
-		return 0;
+		return;
 	/*
-	 * Checkme: Can we simply turn off x2apic here instead of panic?
+	 * Checkme: Can we simply turn off x2APIC here instead of disabling the APIC?
 	 */
-	panic("BIOS has enabled x2apic but kernel doesn't support x2apic, please disable x2apic in BIOS.\n");
+	pr_err("Kernel does not support x2APIC, please recompile with CONFIG_X86_X2APIC.\n");
+	pr_err("Disabling APIC, expect reduced performance and functionality.\n");
+
+	disable_apic = 1;
+	setup_clear_cpu_cap(X86_FEATURE_APIC);
 }
-early_initcall(validate_x2apic);
 
 static inline void try_to_enable_x2apic(int remap_mode) { }
 static inline void __x2apic_enable(void) { }