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

[Jens Axboe <axboe@kernel.dk>]

On 3/31/22 8:33 PM, Kanchan Joshi wrote:
> On Fri, Apr 1, 2022 at 6:55 AM Jens Axboe <axboe@kernel.dk> wrote:
>>
>> On 3/30/22 7:14 AM, Kanchan Joshi wrote:
>>> On Wed, Mar 30, 2022 at 6:32 PM Christoph Hellwig <hch@lst.de> wrote:
>>>>
>>>> On Fri, Mar 25, 2022 at 07:09:21PM +0530, Kanchan Joshi wrote:
>>>>> Ok. If you are open to take new opcode/struct route, that is all we
>>>>> require to pair with big-sqe and have this sorted. How about this -
>>>>
>>>> I would much, much, much prefer to support a bigger CQE.  Having
>>>> a pointer in there just creates a fair amount of overhead and
>>>> really does not fit into the model nvme and io_uring use.
>>>
>>> Sure, will post the code with bigger-cqe first.
>>
>> I can add the support, should be pretty trivial. And do the liburing
>> side as well, so we have a sane base.
> 
>  I will post the big-cqe based work today. It works with fio.
>  It does not deal with liburing (which seems tricky), but hopefully it
> can help us move forward anyway .

Let's compare then, since I just did the support too :-)

Some limitations in what I pushed:

1) Doesn't support the inline completion path. Undecided if this is
super important or not, the priority here for me was to not pollute the
general completion path.

2) Doesn't support overflow. That can certainly be done, only
complication here is that we need 2x64bit in the io_kiocb for that.
Perhaps something can get reused for that, not impossible. But figured
it wasn't important enough for a first run.

I also did the liburing support, but haven't pushed it yet. That's
another case where some care has to be taken to avoid makig the general
path slower.

Oh, it's here, usual branch:

https://git.kernel.dk/cgit/linux-block/log/?h=io_uring-big-sqe

and based on top of the pending 5.18 bits and the current 5.19 bits.

>> Then I'd suggest to collapse a few of the patches in the series,
>> the ones that simply modify or fix gaps in previous ones. Order
>> the series so we build the support and then add nvme support
>> nicely on top of that.
> 
> I think we already did away with patches which were fixing only the
> gaps. But yes, patches still add infra for features incrementally.
> Do you mean having all io_uring infra (async, plug, poll) squashed
> into a single io_uring patch?

At least async and plug, I'll double check on the poll bit.

> On a related note, I was thinking of deferring fixed-buffer and
> bio-cache support for now.

Yes, I think that can be done as a round 2. Keep the current one
simpler.

-- 
Jens Axboe