[PATCH rc] iommufd: Explicitize struct iommu_hwpt_pgfault padding

[PATCH rc] iommufd: Explicitize struct iommu_hwpt_pgfault padding

[Nicolin Chen]

Though the padding could be done by the compiler, add a 32-bit padding
explicitly. Also, change the __u64 to __aligned_u64.

pahole result, before:
struct iommu_hwpt_pgfault {
        __u32     flags;                /*     0     4 */
        __u32     dev_id;               /*     4     4 */
        __u32     pasid;                /*     8     4 */
        __u32     grpid;                /*    12     4 */
        __u32     perm;                 /*    16     4 */

        /* XXX 4 bytes hole, try to pack */

        __u64     addr;                 /*    24     8 */
        __u32     length;               /*    32     4 */
        __u32     cookie;               /*    36     4 */

        /* size: 40, cachelines: 1, members: 8 */
        /* sum members: 36, holes: 1, sum holes: 4 */
        /* last cacheline: 40 bytes */
};

pahole result, after:
struct iommu_hwpt_pgfault {
        __u32      flags;                /*     0     4 */
        __u32      dev_id;               /*     4     4 */
        __u32      pasid;                /*     8     4 */
        __u32      grpid;                /*    12     4 */
        __u32      perm;                 /*    16     4 */
        __u32      __reserved;           /*    20     4 */
        __u64      addr __attribute__((__aligned__(8))); /*    24     8 */
        __u32      length;               /*    32     4 */
        __u32      cookie;               /*    36     4 */

        /* size: 40, cachelines: 1, members: 9 */
        /* forced alignments: 1 */
        /* last cacheline: 40 bytes */
} __attribute__((__aligned__(8)));

Fixes: c714f15860fc ("iommufd: Add fault and response message definitions")
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
---
 include/uapi/linux/iommufd.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/iommufd.h b/include/uapi/linux/iommufd.h
index 34810f6ae2b5..78747b24bd0f 100644
--- a/include/uapi/linux/iommufd.h
+++ b/include/uapi/linux/iommufd.h
@@ -868,6 +868,7 @@ enum iommu_hwpt_pgfault_perm {
  * @pasid: Process Address Space ID
  * @grpid: Page Request Group Index
  * @perm: Combination of enum iommu_hwpt_pgfault_perm
+ * @__reserved: Must be 0.
  * @addr: Fault address
  * @length: a hint of how much data the requestor is expecting to fetch. For
  *          example, if the PRI initiator knows it is going to do a 10MB
@@ -883,7 +884,8 @@ struct iommu_hwpt_pgfault {
 	__u32 pasid;
 	__u32 grpid;
 	__u32 perm;
-	__u64 addr;
+	__u32 __reserved;
+	__aligned_u64 addr;
 	__u32 length;
 	__u32 cookie;
 };
-- 
2.34.1

RE: [PATCH rc] iommufd: Explicitize struct iommu_hwpt_pgfault padding

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@nvidia.com>
> Sent: Saturday, January 18, 2025 10:37 AM
> 
> @@ -868,6 +868,7 @@ enum iommu_hwpt_pgfault_perm {
>   * @pasid: Process Address Space ID
>   * @grpid: Page Request Group Index
>   * @perm: Combination of enum iommu_hwpt_pgfault_perm
> + * @__reserved: Must be 0.

iommufd_fault_fops_read() doesn't follow this statement as
hwpt_fault is on stack with that field uninitialized.

Re: [PATCH rc] iommufd: Explicitize struct iommu_hwpt_pgfault padding

[Nicolin Chen]

On Mon, Jan 20, 2025 at 05:44:27AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@nvidia.com>
> > Sent: Saturday, January 18, 2025 10:37 AM
> > 
> > @@ -868,6 +868,7 @@ enum iommu_hwpt_pgfault_perm {
> >   * @pasid: Process Address Space ID
> >   * @grpid: Page Request Group Index
> >   * @perm: Combination of enum iommu_hwpt_pgfault_perm
> > + * @__reserved: Must be 0.
> 
> iommufd_fault_fops_read() doesn't follow this statement as
> hwpt_fault is on stack with that field uninitialized.

OK. I guess I should update the iommufd_fault_fops_read too.

Thanks
Nic