x86/apic: force phys mode if interrupt remapping is disabled

Cluster mode can only be used with interrupt remapping support, since
the top 16bits of the APIC ID are filled with the cluster ID, and
hence on systems where the physical ID is still smaller than 255 the
cluster ID is not. Force x2APIC to use physical mode if there's no
interrupt remapping support.

Note that this requires a further patch in order to enable x2APIC
without interrupt remapping support.

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

diff --git a/docs/misc/xen-command-line.pandoc b/docs/misc/xen-command-line.pandoc
index cb54a000fcddeb5591755a0058cb6a1323010862..981a5e2381d8ed64d6d8b71bd325b177b4e221f8 100644 (file)
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2322,7 +2322,8 @@ Permit use of x2apic setup for SMP environments.
 ### x2apic_phys (x86)
 > `= <boolean>`
 
-> Default: `true` if **FADT** mandates physical mode, `false` otherwise.
+> Default: `true` if **FADT** mandates physical mode or if interrupt remapping
+>          is not available, `false` otherwise.
 
 In the case that x2apic is in use, this option switches between physical and
 clustered mode.  The default, given no hint from the **FADT**, is cluster

diff --git a/xen/arch/x86/genapic/x2apic.c b/xen/arch/x86/genapic/x2apic.c
index d5a17f10d5cc9944a7454c6d7447af4df2a73aae..1cb16bc10d87dea75a991d75ddd5b79b5febd07a 100644 (file)
--- a/xen/arch/x86/genapic/x2apic.c
+++ b/xen/arch/x86/genapic/x2apic.c
@@ -226,7 +226,21 @@ boolean_param("x2apic_phys", x2apic_phys);
 const struct genapic *__init apic_x2apic_probe(void)
 {
     if ( x2apic_phys < 0 )
-        x2apic_phys = !!(acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL);
+    {
+        /*
+         * Force physical mode if there's no interrupt remapping support: The
+         * ID in clustered mode requires a 32 bit destination field due to
+         * the usage of the high 16 bits to hold the cluster ID.
+         */
+        x2apic_phys = !iommu_intremap ||
+                      (acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL);
+    }
+    else if ( !x2apic_phys && !iommu_intremap )
+    {
+        printk("WARNING: x2APIC cluster mode is not supported without interrupt remapping\n"
+               "x2APIC: forcing phys mode\n");
+        x2apic_phys = true;
+    }
 
     if ( x2apic_phys )
         return &apic_x2apic_phys;