Re: [PATCH V4 00/17] io_uring/ublk: add IORING_OP_FUSED_CMD

[Kanchan Joshi]

On Fri, Mar 24, 2023 at 09:57:51PM +0800, Ming Lei wrote:
> Hello Jens,
>
> Add IORING_OP_FUSED_CMD, it is one special URING_CMD, which has to
> be SQE128. The 1st SQE(master) is one 64byte URING_CMD, and the 2nd
> 64byte SQE(slave) is another normal 64byte OP. For any OP which needs
> to support slave OP, io_issue_defs[op].fused_slave needs to be set as 1,
> and its ->issue() can retrieve/import buffer from master request's
> fused_cmd_kbuf. The slave OP is actually submitted from kernel, part of
> this idea is from Xiaoguang's ublk ebpf patchset, but this patchset
> submits slave OP just like normal OP issued from userspace, that said,
> SQE order is kept, and batching handling is done too.
>
> Please see detailed design in commit log of the 2th patch, and one big
> point is how to handle buffer ownership.
>
> With this way, it is easy to support zero copy for ublk/fuse device.
>
> Basically userspace can specify any sub-buffer of the ublk block request
> buffer from the fused command just by setting 'offset/len'
> in the slave SQE for running slave OP. 

Wondering if this new OP can also be used to do larger IO (than
device limit) on nvme-passthrough?
For example, 1MB IO on NVMe than has 512k or 256K maximum transfer
size.