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.
Regards,
Anthony Liguori
Stefan
