Re: [PATCH RFC 1/1] block/export: FUSE-over-io_uring Support for QEMU FUSE Exports

[Markus Armbruster <armbru@redhat.com>]

Brian Song <hibriansong@gmail.com> writes:

> This work provides an initial implementation of fuse-over-io_uring
> support for QEMU export. According to the fuse-over-io_uring protocol
> specification, the userspace side must create the same number of queues
> as the number of CPUs (nr_cpu), just like the kernel. Currently, each
> queue contains only a single SQE entry, which is used to validate the
> correctness of the fuse-over-io_uring functionality.
>
> All FUSE read and write operations interact with the kernel via io
> vectors embedded in the SQE entry during submission and CQE fetching.
> The req_header and op_payload members of each entry are included as
> parts of the io vector: req_header carries the FUSE operation header,
> and op_payload carries the data payload, such as file attributes in a
> getattr reply, file content in a read reply, or file content being
> written to the FUSE client in a write operation.
>
> At present, multi-threading support is still incomplete. In addition,
> handling connection termination and managing the "drained" state of a
> FUSE block export in QEMU remain as pending work.
>
> Suggested-by: Kevin Wolf <kwolf@redhat.com>
> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Brian Song <hibriansong@gmail.com>

[...]

> diff --git a/docs/tools/qemu-storage-daemon.rst b/docs/tools/qemu-storage-daemon.rst
> index 35ab2d7807..4ec0648e95 100644
> --- a/docs/tools/qemu-storage-daemon.rst
> +++ b/docs/tools/qemu-storage-daemon.rst
> @@ -78,7 +78,7 @@ Standard options:
>  .. option:: --export [type=]nbd,id=<id>,node-name=<node-name>[,name=<export-name>][,writable=on|off][,bitmap=<name>]
>    --export [type=]vhost-user-blk,id=<id>,node-name=<node-name>,addr.type=unix,addr.path=<socket-path>[,writable=on|off][,logical-block-size=<block-size>][,num-queues=<num-queues>]
>    --export [type=]vhost-user-blk,id=<id>,node-name=<node-name>,addr.type=fd,addr.str=<fd>[,writable=on|off][,logical-block-size=<block-size>][,num-queues=<num-queues>]
> -  --export [type=]fuse,id=<id>,node-name=<node-name>,mountpoint=<file>[,growable=on|off][,writable=on|off][,allow-other=on|off|auto]
> +  --export [type=]fuse,id=<id>,node-name=<node-name>,mountpoint=<file>[,growable=on|off][,writable=on|off][,allow-other=on|off|auto][,uring=on|off]
>    --export [type=]vduse-blk,id=<id>,node-name=<node-name>,name=<vduse-name>[,writable=on|off][,num-queues=<num-queues>][,queue-size=<queue-size>][,logical-block-size=<block-size>][,serial=<serial-number>]
>  
>    is a block export definition. ``node-name`` is the block node that should be
> @@ -111,7 +111,13 @@ Standard options:
>    that enabling this option as a non-root user requires enabling the
>    user_allow_other option in the global fuse.conf configuration file.  Setting
>    ``allow-other`` to auto (the default) will try enabling this option, and on
> -  error fall back to disabling it.
> +  error fall back to disabling it. Once ``uring`` is enabled
> +  (off by default), the initialization of FUSE-over-io_uring-related settings
> +  will be performed in the FUSE_INIT request handler. This setup bypasses
> +  the traditional /dev/fuse communication mechanism and instead uses io_uring
> +  for handling FUSE operations.
> +
> +

Drop the additional blank lines, please.

This is user-facing documentation.  Do users care about "the FUSE_INIT
request handler"?

>  
>    The ``vduse-blk`` export type takes a ``name`` (must be unique across the host)
>    to create the VDUSE device.
> diff --git a/qapi/block-export.json b/qapi/block-export.json
> index 9ae703ad01..7d14f3f1ba 100644
> --- a/qapi/block-export.json
> +++ b/qapi/block-export.json
> @@ -184,12 +184,16 @@
>  #     mount the export with allow_other, and if that fails, try again
>  #     without.  (since 6.1; default: auto)
>  #
> +# @uring: If we enable uring option, it will enable FUSE over io_uring
> +#         feature for QEMU FUSE export.  (default: false)
> +#

Missing (since 10.2).

Please format just like everywhere else:

   # @uring: If we enable uring option, it will enable FUSE over
   #     io_uring feature for QEMU FUSE export.  (default: false)


Kernel documentation calls the thing "FUSE-over-io-uring":
https://docs.kernel.org/filesystems/fuse-io-uring.html

The text feels awkward.  Here's my attempt:

   # @uring: Use FUSE-over-io-uring.  (since 10.2; default: false)

>  # Since: 6.0
>  ##
>  { 'struct': 'BlockExportOptionsFuse',
>    'data': { 'mountpoint': 'str',
>              '*growable': 'bool',
> -            '*allow-other': 'FuseExportAllowOther' },
> +            '*allow-other': 'FuseExportAllowOther',
> +            '*uring': 'bool' },
>    'if': 'CONFIG_FUSE' }
>  
>  ##

[...]