[Qemu-devel] Re: RFC: blockdev_add & friends, brief rationale, QMP docs

[Kevin Wolf <kwolf@redhat.com>]

Am 04.06.2010 16:16, schrieb Markus Armbruster:
> Discussion with Christoph and Kevin uncovered yet another issue:
> protocols.  I find it pretty confusing, but let me try to describe it
> anyway; Christoph and Kevin, please correct my errors.
> 
> A host block device has a format.  A format has a name.
> 
> Below the format, it has a stack of protocols.  A protocol has a name
> (with one exception), and may have protocol-specific arguments.
> 
> The most basic (and most commonly used) protocol is for accessing a
> file.  Its argument is a file name.  It doesn't have a name.  Which
> makes for ugly prose, so I'll call it "file".

It does have a name, and surprisingly it's called "file" indeed (defined
at block/raw-posix.c:744 for Linux).

> Stacking protocols is somewhat exotic.  Think of stacking blkdebug on
> top of another protocol, say nbd.

Considering that file is a protocol as well as nbd, it's any blkdebug
use that uses protocol stacking and therefore not that exotic - even
though not the most common case, of course.

> Our abstraction for formats is struct BlockDriver.
> 
> Our abstraction for protocols is also struct BlockDriver.  Except for
> the special protocol "file", but that's detail.

See above, file isn't really special.

> 
> Examples:
> 
> -drive file=foo.qcow2,format=qcow2
> 
>  Format "qcow2", protocol "file" with argument filename "foo.img"

Actually the protocol is guessed here. For this, not all protocols are
considered, it's only between file/host_device/host_cdrom/host_floppy
(these are the protocols implementing bdrv_probe_device, and file as the
default if no other protocol feels responsible)

> -drive file=nbd:unix:/tmp/my_socket,format=raw
> 
>  Format "raw", protocol "nbd" with arguments domain "unix", filename
>  "/tmp/my_socket"
> 
> -drive blkdebug:/tmp/blkdebug.cfg:fat:floppy:rw:/tmp/dir
> 
>  Format not specified (system guesses one), protocol "blkdebug" with
>  argument filename "/tmp/blkdebug.cfg" stacked onto protocol "fat" with
>  arguments floppy true, dirname "/tmp/dir"

These look right to me.

> 
> You see that -drive has a separate option for format, but has protocols
> encoded in option file, in their own mini-language.  Doesn't work for
> arbitrary filenames.  Besides, mini-languages to encode options in
> strings are quite inappropriate for QMP.
> 
> So we need something cleaner for QMP.  Here's a sketch.  Instead of
> 
> - "file": the disk image file to use (json-string, optional)
> - "format": disk format (json-string, optional)
>     - Possible values: "raw", "qcow2", ...
> 
> have
> 
> - "format": disk format (json-string, optional)
>     - Possible values: "raw", "qcow2", ...
> - "protocol": json-array of json-object
>   Each element object has a member "name"
>     - Possible values: "file", "nbd", ...
>   Additional members depend on the value of "name".
>   For "name" = "file":
>     - "file": file name (json-string)
>   For "name" = "nbd":
>     - "domain": address family (json-string, optional)
>         - Possible values: "inet" (default), "unix"
>     - "file": file name (json-string), only with "domain" = "unix"
>     - "host": host name (json-string), only with "domain" = "inet"
>     - "port": port (json-int), only with "domain" = "inet"
>   ...
> 
> You get the idea.
> 
> Comments?

Makes sense.

So blkdebug would define a field "protocol" (json-object) that it uses
to initialize the underlying protocol and we would get the stacking this
way?

Kevin