Re: [PATCH] iommu: Add padding to struct iommu_fault

Jacob Pan
2019-06-12T19:01:41Z

On Wed, 12 Jun 2019 18:59:38 +0100
Jean-Philippe Brucker <jean-philippe.brucker@arm.com> wrote:

> 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];
> };
> };
>

Acked-by: Jacob Pan <jacob.jun.pan@linux.intel.com>

> /**

[Jacob Pan]