cpu/x86/lapic: Add Kconfig choice LAPIC_ACCESS_MODE

Allows compile-time optimisation on platforms that do not wish
to enable runtime checking of X2APIC.

Legacy lapic_cpu_init() is incompatible so there is dependency
on PARALLEL_MP. Also stop_this_cpu() is incompatible, so there
is dependency on !AP_IN_SIPI_WAIT.

Since the code actually lacks enablement of X2APIC (apparently
assuming the blob has done it) and the other small flaws pointed
out in earlier reviews, X2APIC_RUNTIME is not selected per
default on any platform yet.

Change-Id: I8269f9639ee3e89a2c2b4178d266ba2dac46db3f
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/55073
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Wonkyu Kim <wonkyu.kim@intel.com>

src/cpu/x86/Kconfig

@@ -17,6 +17,28 @@ config PARALLEL_MP_AP_WORK
 config LEGACY_SMP_INIT
 	bool
 
+choice LAPIC_ACCESS_MODE
+	prompt "APIC operation mode"
+	default XAPIC_ONLY
+
+config XAPIC_ONLY
+	prompt "Set XAPIC mode"
+	bool
+
+config X2APIC_ONLY
+	prompt "Set X2APIC mode"
+	bool
+	depends on PARALLEL_MP
+	depends on !AP_IN_SIPI_WAIT
+
+config X2APIC_RUNTIME
+	prompt "Support both XAPIC and X2APIC"
+	bool
+	depends on PARALLEL_MP
+	depends on !AP_IN_SIPI_WAIT
+
+endchoice
+
 config UDELAY_LAPIC
 	bool
 	default n

src/cpu/x86/lapic/lapic.c

@@ -3,6 +3,10 @@
 #include <cpu/x86/lapic.h>
 #include <console/console.h>
 
+#if !CONFIG(XAPIC_ONLY)
+#error "BUG: lapic_write_around() needs to be fixed for X2APIC."
+#endif
+
 void lapic_virtual_wire_mode_init(void)
 {
 	/* this is so interrupts work. This is very limited scope --

src/include/cpu/x86/lapic.h

@@ -10,6 +10,12 @@
 
 static inline bool is_x2apic_mode(void)
 {
+	if (CONFIG(XAPIC_ONLY))
+		return false;
+
+	if (CONFIG(X2APIC_ONLY))
+		return true;
+
 	msr_t msr;
 	msr = rdmsr(LAPIC_BASE_MSR);
 	return ((msr.lo & LAPIC_BASE_X2APIC_ENABLED) == LAPIC_BASE_X2APIC_ENABLED);