Re: [Qemu-devel] How to specify the full block driver tree on the CLI ?

[Daniel P. Berrange]

On Thu, Oct 29, 2015 at 09:11:15AM +0100, Kevin Wolf wrote:
> Am 29.10.2015 um 00:58 hat Daniel P. Berrange geschrieben:
> > As previously mentioned, I'm working on support for LUKS full disk
> > encryption in QEMU. I have a simple driver implemented that works
> > on top of plain files. eg I can launch qemu-io thus:
> > 
> >  $ qemu-io /home/berrange/VirtualMachines/demo.luks-aes-cbc-plain-sha256
> > 
> > and it'll probe the luks format & instantiate my "luks" block driver impl
> > on top of the "file" driver. IIUC, I should be able to layer this format
> > driver on top of any of the QEMU block driver backends though. In particular
> > I want to be able to layer it on top of any of the network drivers (RBD,
> > iSCSI and glusterfs).
> 
> This part should work automatically as well if you just use the right
> URL. qemu probes the format even if you're using a non-file protocol.

Ahh, interesting, I guess I should have just tested it rather than
assuming it doesn't work :-)

> > I'm struggling to figure out the right syntax to
> > specify this to QEMU though, using either qemu-io, or the system emulators
> > with the -drive arg.  Are there any docs somewhere about the way to
> > structure the command line arguments to build up a stack of block drivers.
> 
> You have a two options. The one that is universal (that is, it works in
> all places that open an image), but a bit awkward to use manually is the
> json: pseudo-protocol. The "filename" then contains a JSON object of the
> QAPI BlockdevOptions type. The QAPI schema (qapi/block-core.json) is
> probably the best documentation you get. When specifying JSON objects on
> the -drive command line option, don't forget to escape commas by
> doubling.
> 
> For example:
> 
>     qemu-system-x86_64 -drive file='json:{"driver":"luks",,\
>     "secret":"x",,"file":{"driver":"file",,"filename":"test.luks"}}'
> 
> In qemu proper, you can use a dot syntax for -drive instead:
> 
>     qemu-system-x86_64 -drive \
>     driver=luks,\
>     secret=x,\
>     file.driver=file,\
>     file.filename=test.luks
> 
> In qemu-io, you can't use such syntax on the command line, but the open
> command supports an -o option that accepts the same dot syntax.
> 
> Note that qemu-img can't deal with this stuff yet, so you'll have
> trouble creating an image with such a specification. I guess you need to
> create it as a local file first and then use non-qemu tools to copy it
> somewhere where it's exported by rbd, iscsi or gluster.

I wonder if my patches to qemu-io & qemu-img here do the right thing to
make this dot syntax work....

   https://lists.gnu.org/archive/html/qemu-devel/2015-10/msg04382.html
   https://lists.gnu.org/archive/html/qemu-devel/2015-10/msg04375.html

> 
> > I'd like to figure out the following combinations, for qemu-io, qemu-img
> > and system emulator -drive syntax.
> > 
> >  - luks -> file
> >  - qcow2 -> luks -> file
> 
> This is the only case that isn't exactly the same as the example above.
> I guess eventually we'll want to make qemu probe on top of luks, so that
> you can just specify file=foo.qcow2.luks and it works.
> 
> For now, you have to be explicit and nest a level deeper:
> 
>     qemu-system-x86_64 -drive \
>     driver=qcow2,\
>     file.driver=luks,\
>     file.secret=x,\
>     file.file.driver=file,\
>     file.file.filename=test.luks
> 
> Or the same structure in JSON, of course.
> 
> >  - luks -> rbd
> >  - luks -> iscsi
> >  - luks -> glusterfs
> > 
> > Currently the only required QemuOpt for the luks driver is the ID of
> > a secret to provide the password.
> 
> Hope that helps.

Yep, very helpful thanks.

Regards,
Daniel
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