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

[Kanchan Joshi]

On Fri, Mar 11, 2022 at 11:57 AM Christoph Hellwig <hch at lst.de> wrote:
> > 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.
Right. Maybe it is easier for applications if they get to use the same
ioctl opcode/structure that they know well already.

> 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.

Creating a large CQE was my thought too. Gave that another stab.
Dealing with two types of CQE felt nasty to fit in liburing's api-set
(which is cqe-heavy).

Jens: Do you already have thoughts (go/no-go) for this route?

>From all that we discussed, maybe the path forward could be this:
- inline-cmd/big-sqe is useful if paired with big-cqe. Drop big-sqe
for now if we cannot go the big-cqe route.
- use only indirect-cmd as this requires nothing special, just regular
sqe and cqe. We can support all passthru commands with a lot less
code. No new ioctl in nvme, so same semantics. For common commands
(i.e. read/write) we can still avoid updating the result (put_user
cost will go).

Please suggest if we should approach this any differently in v2.

Thanks,
-- 
Kanchan