Re: [Qemu-devel] Re: RFC v2: blockdev_add & friends, brief rationale, QMP docs

[Anthony Liguori]

On 06/16/2010 07:41 AM, Markus Armbruster wrote:
> Kevin Wolf<address@hidden>  writes:
>
>    
> > Am 15.06.2010 15:44, schrieb Avi Kivity:
> >      
> > > On 06/10/2010 08:45 PM, Markus Armbruster wrote:
> > >        
> > > >      * Our config file format is in INI syntax.  QemuOpts correspond to
> > > >        INI sections.  Sections can't be nested, so recursive QemuOpts
> > > >        don't translate.
> > > >
> > > >          
> > > git (and probably others) use
> > >
> > > [a "b"]
> > >       c = d
> > >
> > > for
> > >
> > >      a.b.c=d
> > >
> > >        
> > > >      Examples:
> > > >
> > > >      * Single protocol:
> > > >
> > > >        -blockdev id=blk1,format=raw,protocol=[file,file=fedora.img]
> > > >
> > > >        Requires suitable syntactic sugar to get the simple form (*).
> > > >
> > > >      * blkdebug
> > > >
> > > >        -blockdev id=blk2,format=qcow2,\
> > > >        protocol=[blkdebug,config=test.blkdebug,\
> > > >        protocol=[file,file=test.qcow2]]
> > > >
> > > >      * Avi's mirror:
> > > >
> > > >        -blockdev id=blk3,format=raw,\
> > > >        protocol=[mirror,\
> > > >        [file,file=local.img],\
> > > >        [nbd,domain=unix,sockert=nbd-sock]]
> > > >
> > > > 2. We already have a syntax to specify trees, namely JSON, so use it
> > > >
> > > >      If -blockdev's argument starts with '{', it's a JSON object suitable
> > > >      as argument of blockdev_add in QMP.
> > > >
> > > >      We still provide ordinary QemuOpts syntax for the cases that can be
> > > >      expressed with it, i.e. single protocol.
> > > >
> > > >      I figure we'd want syntactic sugar for blkdebug, to permit its use
> > > >      from the command line without having to resort to JSON.
> > > >
> > > >          
> > > Might be nice as a general extension to QemuOpts.
> > >        
> > I agree.
> >
> >      
> > >        
> > > > 3. Stack protocols through named references
> > > >
> > > >      The first protocol is "inlined" into -blockdev.  Any further
> > > >      protocols need to be referenced by name.
> > > >
> > > >      Best explained by example:
> > > >
> > > >      * Single protocol:
> > > >
> > > >        -blockdev id=blk1,format=raw,protocol=file,file=fedora.img
> > > >
> > > >        To get the simple form (*), make protocol optional with a suitable
> > > >        default.
> > > >
> > > >      * blkdebug
> > > >
> > > >        -blockdev id=blk2,format=qcow2,protocol=blkdebug,config=test.blkdebug,\
> > > >        base=blk2-base
> > > >        -blockproto id=blk2-base,protocol=file,file=test.qcow2
> > > >
> > > >      * Avi's mirror:
> > > >
> > > >        -blockdev id=blk3,format=raw,protocol=mirror,\
> > > >        base=blk3-base1,base=blk3=base2
> > > >        -blockproto id=blk3-base1,protocol=file,file=local.img
> > > >        -blockproto id=blk3-base2,protocol=nbd,domain=unix,sockert=nbd-sock
> > > >
> > > >      Anything but a single protocol becomes pretty verbose.  Syntactic
> > > >      sugar for the blkdebug case would be possible; not sure it's worth
> > > >      it.
> > > >
> > > >      No QemuOpts syntax changes.  INI can handle this just fine.
> > > >
> > > >
> > > >          
> > > Looks like the least painful option as no new infrastructure is needed.
> > > I'd go with this.
> > >        
> > But it's painful to type for the user. After all -blockdev on the
> > command line is for the user, as tools should use QMP. Also note that
> > this syntax mixes format and protocol options on one line which I
> > consider confusing at best.
> >
> > As I told Markus already in private before he posted this, I prefer the
> > bracket solution for its clarity and simplicity, even though it comes at
> > the cost of having additional characters that need to be escaped.
> >      
> I dont't think 1. is less painful than 3.  Let's compare the two:
>
> * Single protocol: identical with suitable syntactical sugar, namely
>
>        -blockdev id=blk1,file=fedora.img
>    

First, let me say that -blockdev is not something that I believe is 
targeted at users.  It's incredible unfair for us to expect a user to type:

-blockdev id=blk1,file=fedora.img -device ide-drive,drive=blk1,bus=0,unit=0

Instead of:

-hda fedora.img

I had to look up the device syntax just to write that.  There's no way 
users are going to do this.  We should drop any notion of syntactical 
sugar IMHO.  -blockdev is for management tools, scripts, and as an 
infrastructure for config files.

>    Unsugared it's
>
>        -blockdev id=blk1,format=raw,protocol=[file,file=fedora.img]
>    vs.
>
>        -blockdev id=blk1,format=raw,protocol=file,file=fedora.img
>    

Specifying nesting in a single option is a bad idea.  It should be:

-blockdev id=blk1,format=raw,protocol=blk2  \
-blockdev id=blk2,file=fedora.img

But honestly, I'm thoroughly confused about the distinction between 
protocol and format.  I had thought that protocols were a type of format 
and I'm not sure why we're making a distinction.

>    I sure prefer the latter.  The brackets look like noise.  You need to
>    understand protocol stacking for them to make any sense.
>
>    Regarding confusion caused by mixing format and protocol options: yes,
>    the brackets force you to distinguish between protocol options and
>    other options.  But I doubt that'll reduce confusion here.  Either you
>    understand protocols.  Then I doubt you need brackets to unconfuse
>    you.  Or you don't understand protocols.  Then whether to put an
>    option inside or outside the brackets is voodoo.
>    

If the above is necessary just to create a raw image, then we're doing 
something wrong in the block layer.  If should be possible to just say:

-blockdev id=blk1,format=raw,file=fedora.img

Regards,

Anthony Liguori