Re: [PATCHSET v2 0/8] Add support for mixed sized CQEs

[Jens Axboe <axboe@kernel.dk>]

On 8/21/25 11:40 AM, Keith Busch wrote:
> On Thu, Aug 21, 2025 at 11:12:28AM -0600, Jens Axboe wrote:
>>
>> For the SQE case, I think it's a bit different. At least the cases I
>> know of, it's mostly 100% 64b SQEs or 128b SQEs. I'm certainly willing
>> to be told otherwise! Because that is kind of the key question that
>> needs answering before even thinking about doing that kind of work.
> 
> The main use case I can think of is if an application allocates one ring
> for uring_cmd with the 128b SQEs, and then a separate ring for normal
> file and network stuff. Mixed SQE's would allow that application to have
> just one ring without being wasteful, but I'm just not sure if the
> separate rings is undesirable enough to make the effort worth it.

Indeed! And like Caleb mentioned, their use case already does this in
fact, just passthrough with housekeeping buffer commands.

>> But yes, it could be supported, and Keith (kind of) signed himself up to
>> do that. One oddity I see on that side is that while with CQE32 the
>> kernel can manage the potential wrap-around gap, for SQEs that's
>> obviously on the application to do. That could just be a NOP or
>> something like that, but you do need something to fill/skip that space.
>> I guess that could be as simple as having an opcode that is simply "skip
>> me", so on the kernel side it'd be easy as it'd just drop it on the
>> floor. You still need to app side to fill one, however, and then deal
>> with "oops SQ ring is now full" too.
> 
> Yep, I think it's doable, and your implementation for mixed CQEs
> provides a great reference. I trust we can get liburing using it
> correctly, but would be afraid for anyone not using the library.

That's the same with the mixed CQEs though - as you can see from the
liburing changes, it's really not hard to support. If you're using the
raw interface, well then things are already a bit more complicated for
you. Not too worried about that use case.

-- 
Jens Axboe