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

[Stefan Hajnoczi]

On Thu, Jul 28, 2011 at 10:29 AM, Stefan Hajnoczi <address@hidden> wrote:
> On Wed, Jul 27, 2011 at 5:02 PM, Anthony Liguori <address@hidden> wrote:
>> 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.
>
> Use query-block and see if 'hostcache' is there.  If yes, then the
> hostcache parameter is available.  If we allow BlockDrivers to have
> their own runtime parameters then query-commands does not tell you
> anything because the specific BlockDriver may or may not support that
> runtime parameter - you need to use query-block.

Let's reach agreement here.  The choices are:

1. Top-level block_set command.  Supported parameters are discovered
by looking query-block output.

2. Top-level command for each parameter (e.g. block_set_hostcache).
Supported parameters are easily discoverable via query-commands.  If
individual block devices support different sets of parameters then
they may have to return -ENOTSUPP.

I like the block_set approach.

Anthony, Kevin, Supriya: Any thoughts?

Stefan