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

From: Sagi Grimberg <sagi@grimberg.me>
To: Bart Van Assche <bvanassche@acm.org>,
	Mike Christie <michael.christie@oracle.com>,
	Gabriel Krisman Bertazi <krisman@collabora.com>
Cc: Hannes Reinecke <hare@suse.de>,
	lsf-pc@lists.linux-foundation.org, linux-block@vger.kernel.org
Subject: Re: [LSF/MM/BPF TOPIC] block drivers in user space
Date: Tue, 15 Mar 2022 10:04:49 +0200	[thread overview]
Message-ID: <c6d862ce-9f3d-d89f-dee5-fd40b096dbf2@grimberg.me> (raw)
In-Reply-To: <0b85385b-e8cf-2ab3-ce22-c63d4346cc16@acm.org>

>> 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.