Re: [Qemu-devel] [V5 Patch 3/4]Qemu: Command "block_set" for dynamic block params change

[Anthony Liguori]

On 07/27/2011 09:31 AM, Stefan Hajnoczi wrote:
> On Wed, Jul 27, 2011 at 1:58 PM, Anthony Liguori<address@hidden>  wrote:
> > > Index: qemu/hmp-commands.hx
> > > ===================================================================
> > > --- qemu.orig/hmp-commands.hx
> > > +++ qemu/hmp-commands.hx
> > > @@ -70,6 +70,20 @@ but should be used with extreme caution.
> > >   resizes image files, it can not resize block devices like LVM volumes.
> > >   ETEXI
> > >
> > > +    {
> > > +        .name       = "block_set",
> > > +        .args_type  = "device:B,device:O",
> > > +        .params     = "device [prop=value][,...]",
> > > +        .help       = "Change block device parameters
> > > [hostcache=on/off]",
> > > +        .user_print = monitor_user_noop,
> > > +        .mhandler.cmd_new = do_block_set,
> > > +    },
> > > +STEXI
> > > address@hidden block_set @var{config}
> > > address@hidden block_set
> > > +Change block device parameters (eg: hostcache=on/off) while guest is
> > > running.
> > > +ETEXI
> > > +
> >
> > block_set_hostcache() please.
> >
> > Multiplexing commands is generally a bad idea.  It weakens typing.  In the
> > absence of a generic way to set block device properties, implementing
> > properties as generic in the QMP layer seems like a bad idea to me.
>
> The idea behind block_set was to have a unified interface for changing
> block device parameters at runtime.  This prevents us from reinventing
> new commands from scratch.  For example, block I/O throttling is
> already queued up to add run-time parameters.
>
> Without a unified command we have a bulkier QMP/HMP interface,
> duplicated code, and possibly inconsistencies in syntax between the
> commands.  Isn't the best way to avoid these problems a unified
> interface?
>
> I understand the lack of type safety concern but in this case we
> already have to manually pull parsed arguments (i.e. cast to specific
> types and deal with invalid input).  To me this is a reason *for*
> using a unified interface like block_set.

Think about it from a client perspective.  How do I determine which 
properties are supported by this version of QEMU?  I have no way to 
identify programmatically what arguments are valid for block_set.

OTOH, if you have strong types like block_set_hostcache, query-commands 
tells me exactly what's supported.

Regards,

Anthony Liguori

> Stefan