[PATCH 05/17] nvme: wire-up support for async-passthru on char-device.

[Kanchan Joshi]

On Tue, Mar 15, 2022 at 11:02:30AM +0200, Sagi Grimberg wrote:
>
>>>>+int nvme_ns_head_chr_async_cmd(struct io_uring_cmd *ioucmd)
>>>>+{
>>>>+     struct cdev *cdev = file_inode(ioucmd->file)->i_cdev;
>>>>+     struct nvme_ns_head *head = container_of(cdev, struct nvme_ns_head, cdev);
>>>>+     int srcu_idx = srcu_read_lock(&head->srcu);
>>>>+     struct nvme_ns *ns = nvme_find_path(head);
>>>>+     int ret = -EWOULDBLOCK;
>>>>+
>>>>+     if (ns)
>>>>+             ret = nvme_ns_async_ioctl(ns, ioucmd);
>>>>+     srcu_read_unlock(&head->srcu, srcu_idx);
>>>>+     return ret;
>>>>+}
>>>
>>>No one cares that this has no multipathing capabilities what-so-ever?
>>>despite being issued on the mpath device node?
>>>
>>>I know we are not doing multipathing for userspace today, but this
>>>feels like an alternative I/O interface for nvme, seems a bit cripled
>>>with zero multipathing capabilities...
>>
>>Multipathing is on the radar. Either in the first cut or in
>>subsequent. Thanks for bringing this up.
>
>Good to know...
>
>>So the char-node (/dev/ngX) will be exposed to the host if we enable
>>controller passthru on the target side. And then the host can send
>>commands using uring-passthru in the same way.
>
>Not sure I follow...

Doing this on target side:
echo -n /dev/nvme0 > /sys/kernel/config/nvmet/subsystems/testnqn/passthru/device_path
echo 1 > /sys/kernel/config/nvmet/subsystems/testnqn/passthru/enable

>>May I know what are the other requirements here.
>
>Again, not sure I follow... The fundamental capability is to
>requeue/failover I/O if there is no I/O capable path available...

That is covered I think, with nvme_find_path() at places including the
one you highlighted above.