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

[Auger Eric <eric.auger@redhat.com>]

Hi Jean,

On 6/12/19 7:59 PM, Jean-Philippe Brucker 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>
Reviewed-by: Eric Auger <eric.auger@redhat.com>

Thanks

Eric

> ---
>  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];
>  	};
>  };
>  
>  /**
> 
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