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

[Stefan Hajnoczi]

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.

Stefan