[PATCH v2 1/6] x86/Kconfig: add option for default x2APIC destination mode

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

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@xxxxxxxxxx>
---
Changes since v1:
 - Use a boolean rather than a choice.
 - Expand to X2APIC_PHYSICAL.
---
TBH I wasn't sure whether to keep X2APIC_PHYSICAL or X2APIC_LOGICAL as
the Kconfig option, went with X2APIC_PHYSICAL because that's the
define the code was already using.
---
 xen/arch/x86/Kconfig          | 18 ++++++++++++++++++
 xen/arch/x86/genapic/x2apic.c |  6 ++++--
 2 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/Kconfig b/xen/arch/x86/Kconfig
index 1e31edc99f..6bed72b791 100644
--- a/xen/arch/x86/Kconfig
+++ b/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
 
diff --git a/xen/arch/x86/genapic/x2apic.c b/xen/arch/x86/genapic/x2apic.c
index de5032f202..7dfc793514 100644
--- a/xen/arch/x86/genapic/x2apic.c
+++ b/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 )
-- 
2.36.1