[Xen-devel] [PATCH v2 2/4] x86/apic: force phys mode if interrupt remapping is disabled

[Roger Pau Monne <roger.pau@citrix.com>]

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>
---
Changes since v1:
 - New in this version.
---
 xen/arch/x86/genapic/x2apic.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/xen/arch/x86/genapic/x2apic.c b/xen/arch/x86/genapic/x2apic.c
index d5a17f10d5..7e32ee22ff 100644
--- a/xen/arch/x86/genapic/x2apic.c
+++ b/xen/arch/x86/genapic/x2apic.c
@@ -228,6 +228,14 @@ const struct genapic *__init apic_x2apic_probe(void)
     if ( x2apic_phys < 0 )
         x2apic_phys = !!(acpi_gbl_FADT.flags & ACPI_FADT_APIC_PHYSICAL);
 
+    if ( !x2apic_phys && !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;
+
     if ( x2apic_phys )
         return &apic_x2apic_phys;
 
-- 
2.24.0


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