[Xen-devel] [PATCH v3 2/4] x86/apic: force phys mode if interrupt remapping is disabled

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

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@xxxxxxxxxx>
---
Changes since v2:
 - Update command line doc.
 - Fix logic to set x2apic_phys if user has specified a value.
 - Force phys mode if no interrupt remapping support.

Changes since v1:
 - New in this version.
---
 docs/misc/xen-command-line.pandoc |  3 ++-
 xen/arch/x86/genapic/x2apic.c     | 18 +++++++++++++++++-
 2 files changed, 19 insertions(+), 2 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index 891d2d439f..d9495ef6b9 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -2303,7 +2303,8 @@ 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 d5a17f10d5..79b6c07329 100644
--- a/xen/arch/x86/genapic/x2apic.c
+++ b/xen/arch/x86/genapic/x2apic.c
@@ -226,7 +226,23 @@ 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);
+    {
+        if ( !iommu_intremap )
+            /*
+             * 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 store the cluster ID.
+             */
+            x2apic_phys = true;
+        else
+            x2apic_phys = !!(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;
-- 
2.24.0


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