Re: nvme_passthru_cmd64 has a 4 byte hole

[Charles Machalow <csm10495@gmail.com>]

Ah yes. I'd greatly prefer that. That way the new structure can likely
even be used with the old Ioctls and the 2nd u32 should just stay not
get set if used in the old flow

- Charlie Scott Machalow

On Tue, Oct 29, 2019 at 11:32 PM Keith Busch <kbusch@kernel.org> wrote:
>
> On Tue, Oct 29, 2019 at 09:43:20PM -0700, Charles Machalow wrote:
> > Hey all,
> >
> > I noticed (via some debug) that nvme_passthru_cmd64 seems to have a 4
> > byte hole that isn't currently accounted for in the struct:
> > From pahole:
> >
> > struct nvme_passthru_cmd64 {
> >         __u8                       opcode;               /*     0     1 */
> >         __u8                       flags;                /*     1     1 */
> >         __u16                      rsvd1;                /*     2     2 */
> >         __u32                      nsid;                 /*     4     4 */
> >         __u32                      cdw2;                 /*     8     4 */
> >         __u32                      cdw3;                 /*    12     4 */
> >         __u64                      metadata;             /*    16     8 */
> >         __u64                      addr;                 /*    24     8 */
> >         __u32                      metadata_len;         /*    32     4 */
> >         __u32                      data_len;             /*    36     4 */
> >         __u32                      cdw10;                /*    40     4 */
> >         __u32                      cdw11;                /*    44     4 */
> >         __u32                      cdw12;                /*    48     4 */
> >         __u32                      cdw13;                /*    52     4 */
> >         __u32                      cdw14;                /*    56     4 */
> >         __u32                      cdw15;                /*    60     4 */
> >         /* --- cacheline 1 boundary (64 bytes) --- */
> >         __u32                      timeout_ms;           /*    64     4 */
> >
> >         /* XXX 4 bytes hole, try to pack */
> >
> >         __u64                      result;               /*    72     8 */
> >
> >         /* size: 80, cachelines: 2, members: 18 */
> >         /* sum members: 76, holes: 1, sum holes: 4 */
> >         /* last cacheline: 16 bytes */
> > };
> >
> > (Since I doubt we can functionally change the structure at this
> > point...)
>
> Well, there is no official kernel release using this structure,
> so I suppose even this late in the rc cycle, we can change it with
> appropriate justification. I think an alternative to padding is to make
> this a __u32[2] field, which should also be backward compatible with
> the 4 byte result passthrough.

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