[PATCH] iommu: Add padding to struct iommu_fault

Jean-Philippe Brucker
2019-06-12T18:06:23Z

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>
---
include/uapi/linux/iommu.h | 2 ++
1 file changed, 2 insertions(+)

diff --git a/include/uapi/linux/iommu.h b/include/uapi/linux/iommu.h
index f45d8e9e59c3..fc00c5d4741b 100644
--- a/include/uapi/linux/iommu.h
+++ b/include/uapi/linux/iommu.h
@@ -105,15 +105,17 @@ struct iommu_fault_page_request {
* @type: fault type from &enum iommu_fault_type
* @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];
};
};

/**
--
2.21.0