Re: [LSF/MM/BPF BOF] Userspace command abouts

[Chaitanya Kulkarni <chaitanyak@nvidia.com>]

(+Martin, Damien)

On 2/23/2023 7:35 AM, Sagi Grimberg wrote:
> 
>>>>> I did not understand what is the relationship between aborts and CDL.
>>>>> Sounds to me that this would tie in to something like Time Limited 
>>>>> Error
>>>>> Recovery (TLER) and LR bit set based on ioprio?
>>>>>
>>>>> I am unclear where do aborts come into play here.
>>>>
>>>> CDL: Command Duration Limits
>>>>
>>>> One use case is reading from storage for audio visual output.
>>>> An application only wants to wait so long (e.g. one or two frames
>>>> on the video output) before it wants to forget about the current
>>>> read (i.e. "abort" it) and move onto the next read. An alert viewer
>>>> might notice a momentary freeze frame.
>>>>
>>>> The SCSI CDL mechanism uses the DL0, DL1 and DL2 bits in the 
>>>> READ(16,32)
>>>> commands. CDL also depends on the CDLP and RWCDLP fields in the
>>>> REPORT SUPPORTED OPERATION CODES command and one of the CDL
>>>> mode pages. So there may be some additional "wiring" needed in the
>>>> SCSI subsystem.
>>>
>>> I still don't understand where issuing aborts from userspace come into
>>> play here...
>>
>> The only connection is that aborts are obsolete and unnecessary if
>> you have a working CDL implementation.
> 
> OK, that makes sense. Indeed I *think* that nvme can support CDL and if
> this is useful for userspace then this is an interesting path to take.

I do think that we should work on CDL for NVMe as it will solve some of
the timeout related problems effectively than using aborts or any other
mechanism.

-ck