[PATCH 17/17] nvme: enable non-inline passthru commands

[Christoph Hellwig]

On Fri, Mar 11, 2022 at 12:13:24AM +0530, Kanchan Joshi wrote:
> Problem is, the inline facility does not go very well with this
> particular nvme-passthru ioctl (NVME_IOCTL_IO64_CMD).

And it doesn't have to, because there is absolutely no need to reuse
the existing structures!  Quite to the contrary, trying to reuse the
structure and opcode makes things confusing as hell.

> And that's because this ioctl requires additional "__u64 result;" to
> be updated within "struct nvme_passthru_cmd64".
> To update that during completion, we need, at the least, the result
> field to be a pointer "__u64 result_ptr" inside the struct
> nvme_passthru_cmd64.
> Do you see that is possible without adding a new passthru ioctl in nvme?

We don't need a new passthrough ioctl in nvme.  We need to decouple the
uring cmd properly.  And properly in this case means not to add a
result pointer, but to drop the result from the _input_ structure
entirely, and instead optionally support a larger CQ entry that contains
it, just like the first patch does for the SQ.