intel-iommu: Fix boot inside 64bit virtualbox with io-apic disabled

Commit 074835f0143b83845af5044af2739c52c9f53808 ("intel-iommu: Fix
kernel hand if interrupt remapping disabled in BIOS") is adding a check
for interrupt remapping disabled and is dereferencing the dmar_tbl
pointer without checking its value.

Unfortunately, this value is null when booting inside a 64bit virtual
box guest with io-apic disabled, leading to a crash. With a check on it,
the guest is now booting. It's triggering a WARN() in
clockevent_delta2ns but it's better than not booting at all and allows
the user to see there's something wrong on their io-apic setup.

Signed-off-by: Arnaud Patard <apatard@mandriva.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/drivers/pci/dmar.c b/drivers/pci/dmar.c
index edc5f002e055..7771b2dd5978 100644
--- a/drivers/pci/dmar.c
+++ b/drivers/pci/dmar.c
@@ -1466,5 +1466,7 @@ int __init dmar_ir_support(void)
 {
         struct acpi_table_dmar *dmar;
         dmar = (struct acpi_table_dmar *)dmar_tbl;
+        if (!dmar)
+                return 0;
         return dmar->flags & 0x1;
 }