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

[Christoph Hellwig]

On Tue, Mar 22, 2022 at 10:40:27PM +0530, Kanchan Joshi wrote:
> 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.

I disagree.  Reusing the same opcode and/or structure for something
fundamentally different creates major confusion.  Don't do it.

> >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.

Personally I think larger SQEs and CQEs are the only sensible interface
here.  Everything else just fails like a horrible hack I would not want
to support in NVMe.