On Wed, Jul 27, 2011 at 5:02 PM, Anthony Liguori <anth...@codemonkey.ws> wrote:
> On 07/27/2011 09:31 AM, Stefan Hajnoczi wrote:
>>
>> On Wed, Jul 27, 2011 at 1:58 PM, Anthony Liguori<anth...@codemonkey.ws>
>> 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
>>>> +@item block_set @var{config}
>>>> +@findex 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.
Stefan
