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

[Supriya Kannery <supriyak@in.ibm.com>]

On 07/27/2011 09:32 PM, Anthony Liguori wrote:
> On 07/27/2011 09:31 AM, Stefan Hajnoczi wrote:
>> On Wed, Jul 27, 2011 at 1:58 PM, Anthony
>> Liguori<anthony@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.
>

    User can programmatically find out valid parameters for
block_set. Internally, validation of prop=value is done against -drive
parameter list and then, only the valid/implemented commands (for now, 
hostcache) are accepted from that list. Please find execution output 
attached:
========================================================================
(qemu) info block
ide0-hd0: removable=0 hostcache=1 file=../rhel6-32.qcow2 ro=0 drv=qcow2 
encrypted=0
floppy0: removable=1 locked=0 hostcache=0 file=test.img ro=0 drv=raw 
encrypted=0
ide1-cd0: removable=1 locked=0 hostcache=1 [not inserted]
sd0: removable=1 locked=0 hostcache=1 [not inserted]
(qemu) block
block_resize  block_set     block_passwd
(qemu) block_set
block_set: block device name expected
(qemu) block
block_resize  block_set     block_passwd
(qemu) help block_set
block_set device [prop=value][,...] -- Change block device parameters 
[hostcache=on/off]
(qemu) block_set ide
Device 'ide' not found
(qemu) block_set ide0-hd0
Usage: block_set device [prop=value][,...]
(qemu) block_set ide0-hd0 cache
Invalid parameter 'cache'
(qemu) block_set ide0-hd0 cache=on
Parameter 'hostcache' expects on/off
(qemu) block_set ide0-hd0 hostcache=on
(qemu) block_set ide0-hd0 hostcache=off
(qemu) info block
ide0-hd0: removable=0 hostcache=0 file=../rhel6-32.qcow2 ro=0 drv=qcow2 
encrypted=0
floppy0: removable=1 locked=0 hostcache=0 file=test.img ro=0 drv=raw 
encrypted=0
ide1-cd0: removable=1 locked=0 hostcache=1 [not inserted]
sd0: removable=1 locked=0 hostcache=1 [not inserted]
========================================================================

  When we add further parameters, "usage" string can be enhanced to
include those parameters for informing user.

- Thanks, Supriya

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