iommu: Add padding to struct iommu_fault

Ease future extensions of struct iommu_fault_page_request and struct
iommu_fault_unrecoverable by adding a few bytes of padding. That way, a
new field can be added to either of these structures by simply introducing
a new flag. To extend it after the size limit is reached, a new fault
reporting structure will have to be negotiated with userspace.

With 56 bytes of padding, the total size of iommu_fault is 64 bytes and
fits in a cache line on a lot of contemporary machines, while providing 16
and 24 bytes of extension to structures iommu_fault_page_request and
iommu_fault_unrecoverable respectively.

Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>

include/uapi/linux/iommu.h

@@ -106,13 +106,15 @@ struct iommu_fault_page_request {
  * @padding: reserved for future use (should be zero)
  * @event: fault event, when @type is %IOMMU_FAULT_DMA_UNRECOV
  * @prm: Page Request message, when @type is %IOMMU_FAULT_PAGE_REQ
+ * @padding2: sets the fault size to allow for future extensions
  */
 struct iommu_fault {
 	__u32	type;
 	__u32	padding;
 	union {
 		struct iommu_fault_unrecoverable event;
 		struct iommu_fault_page_request prm;
+		__u8 padding2[56];
 	};
 };