Re: [LSF/MM/BPF TOPIC] block drivers in user space

[Sagi Grimberg <sagi@grimberg.me>]

>> So I hear there is a desire to do this. So I think we should list the
>> use-cases for this first because that would lead to different design
>> choices.. For example one use-case is just to send read/write/flush
>> to userspace, another may want to passthru nvme commands to userspace
>> and there may be others...
> 
> (resending my reply without truncating the Cc-list)
> 
> Hi Sagi,
> 
> Haven't these use cases already been mentioned in the email at the start 
> of this thread? The use cases I am aware of are implementing 
> cloud-specific block storage functionality and also block storage in 
> user space for Android. Having to parse NVMe commands and PRP or SGL 
> lists would be an unnecessary source of complexity and overhead for 
> these use cases. My understanding is that what is needed for these use 
> cases is something that is close to the block layer request interface 
> (REQ_OP_* + request flags + data buffer).

pasting my response here as well:

Well, I can absolutely think of a use-case that will want raw nvme
commands and leverage vendor specific opcodes for applications like
computational storage or what not...

I would say that all the complexity of handling nvme commands in
userspace would be handle in a properly layered core stack with
pluggable backends that can see a simplified interface if they want, or
see a full passthru command.