x86/Kconfig: add option for default x2APIC destination mode

Allow setting the default x2APIC destination mode from Kconfig to
Physical.

Note the default destination mode is still Logical (Cluster) mode.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/Kconfig

@@ -226,6 +226,24 @@ config XEN_ALIGN_2M
 
 endchoice
 
+config X2APIC_PHYSICAL
+	bool "x2APIC Physical Destination mode"
+	help
+	  Use x2APIC Physical Destination mode by default when available.
+
+	  When using this mode APICs are addressed using the Physical
+	  Destination mode, which allows using all dynamic vectors on each
+	  CPU independently.
+
+	  Physical Destination has the benefit of having more vectors available
+	  for external interrupts, but it also makes the delivery of multi
+	  destination inter processor interrupts (IPIs) slightly slower than
+	  Logical Destination mode.
+
+	  The mode when this option is not selected is Logical Destination.
+
+	  If unsure, say N.
+
 config GUEST
 	bool
 

xen/arch/x86/genapic/x2apic.c

@@ -228,7 +228,7 @@ static struct notifier_block x2apic_cpu_nfb = {
    .notifier_call = update_clusterinfo
 };
 
-static s8 __initdata x2apic_phys = -1; /* By default we use logical cluster mode. */
+static int8_t __initdata x2apic_phys = -1;
 boolean_param("x2apic_phys", x2apic_phys);
 
 const struct genapic *__init apic_x2apic_probe(void)
@@ -241,7 +241,9 @@ const struct genapic *__init apic_x2apic_probe(void)
          * the usage of the high 16 bits to hold the cluster ID.
          */
         x2apic_phys = !iommu_intremap ||
-                      (acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL);
+                      (acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL) ||
+                      (IS_ENABLED(CONFIG_X2APIC_PHYSICAL) &&
+                       !(acpi_gbl_FADT.flags & ACPI_FADT_APIC_CLUSTER));
     }
     else if ( !x2apic_phys )
         switch ( iommu_intremap )