Re: [Qemu-devel] [PATCH 05/10] QemuOpts: add -set option

[Gerd Hoffmann]

On 07/31/09 19:01, Anthony Liguori wrote:
> Gerd Hoffmann wrote:
> > One use case will be file for drives (no filename quoting issues), i.e.
> >
> > -drive id=test,if=virtio
> > -set drive.test.file=/vmdisk/test-virtio.img
> >
> > It will work for any other option (assuming handled by QemuOpts) though.
> > Except for id= for obvious reasons ;).
>
> How can we make it work for id?

Do we want to?

> That is, it would be good to be able to fully define the drive via -set.
> I think this is necessary to introduce a config file.

Here is a short writeup how IDs are handled in the current implementation:

-drive params
  creates a new drive without id.  You can't use -set with that one.

-drive params,id=foo
  creates a new drive named 'foo'.  Trying to create another one named
  'foo' doesn't work, qemu will complain that it already exists.

-set drive.foo.param=value
  configure the (existing) drive foo.

So you need '-drive id=something' as minimum to create a new drive, then 
you can set all parameters via -set if you want.

Ordering of the command line options is important, the -drive switch 
which creates the new drive must come first.


This all isn't set in stone, it can be changed without too much effort. 
 We can make -set silently create a new drive in case the used id 
doesn't exist.  We can allow two -drive switches with the same id, so 
you could do -drive id=foo,if=scsi -drive id=foo,unit=3 and have the 
same effect as -drive id=foo,id=scsi,unit=3.  I just found the current 
behavior most useful for the command line.  If you mistype an ID you 
most likely get a useful error message from the parser for example.


A config file is a completely different story, the config file parser 
can implement different behavior.  It also depends on the config file 
format of course.  We could use git-style:

[drive "foo"]
  if=scsi
  unit=3
  file=/path/to/image

In that case we don't have that problem in the first place ;)

We could use somthing simliar to -set, i.e.

drive.foo.if = scsi
drive.foo.unit = 4
drive.foo.file = /path/to/image

Then the parser will of course create drive "foo" when it finds config 
entries for it.

cheers,
  Gerd