x86/apic: Fix x2apic enablement without interrupt remapping

Currently, Linux as a hypervisor guest will enable x2apic only if there are
no CPUs present at boot time with an APIC ID above 255.

Hotplugging a CPU later with a higher APIC ID would result in a CPU which
cannot be targeted by external interrupts.

Add a filter in x2apic_apic_id_valid() which can be used to prevent such
CPUs from coming online, and allow x2apic to be enabled even if they are
present at boot time.

Fixes: ce69a784 ("x86/apic: Enable x2APIC without interrupt remapping under KVM")
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201024213535.443185-2-dwmw2@infradead.org

arch/x86/include/asm/apic.h

@@ -259,6 +259,7 @@ static inline u64 native_x2apic_icr_read(void)
 
 extern int x2apic_mode;
 extern int x2apic_phys;
+extern void __init x2apic_set_max_apicid(u32 apicid);
 extern void __init check_x2apic(void);
 extern void x2apic_setup(void);
 static inline int x2apic_enabled(void)

arch/x86/kernel/apic/apic.c

@@ -1841,20 +1841,22 @@ static __init void try_to_enable_x2apic(int remap_mode)
 		return;
 
 	if (remap_mode != IRQ_REMAP_X2APIC_MODE) {
-		/* IR is required if there is APIC ID > 255 even when running
-		 * under KVM
+		/*
+		 * Using X2APIC without IR is not architecturally supported
+		 * on bare metal but may be supported in guests.
 		 */
-		if (max_physical_apicid > 255 ||
-		    !x86_init.hyper.x2apic_available()) {
+		if (!x86_init.hyper.x2apic_available()) {
 			pr_info("x2apic: IRQ remapping doesn't support X2APIC mode\n");
 			x2apic_disable();
 			return;
 		}
 
 		/*
-		 * without IR all CPUs can be addressed by IOAPIC/MSI
-		 * only in physical mode
+		 * Without IR, all CPUs can be addressed by IOAPIC/MSI only
+		 * in physical mode, and CPUs with an APIC ID that cannnot
+		 * be addressed must not be brought online.
 		 */
+		x2apic_set_max_apicid(255);
 		x2apic_phys = 1;
 	}
 	x2apic_enable();

arch/x86/kernel/apic/x2apic_phys.c

@@ -8,6 +8,12 @@
 int x2apic_phys;
 
 static struct apic apic_x2apic_phys;
+static u32 x2apic_max_apicid __ro_after_init;
+
+void __init x2apic_set_max_apicid(u32 apicid)
+{
+	x2apic_max_apicid = apicid;
+}
 
 static int __init set_x2apic_phys_mode(char *arg)
 {
@@ -98,6 +104,9 @@ static int x2apic_phys_probe(void)
 /* Common x2apic functions, also used by x2apic_cluster */
 int x2apic_apic_id_valid(u32 apicid)
 {
+	if (x2apic_max_apicid && apicid > x2apic_max_apicid)
+		return 0;
+
 	return 1;
 }