Re: [Qemu-devel] GSoC mentor summit QEMU users session

[Anthony Liguori <anthony@codemonkey.ws>]

On 11/02/2011 01:34 PM, Alexander Graf wrote:
> Anthony Liguori wrote:
>> On 11/02/2011 01:17 PM, Jan Kiszka wrote:
>>> On 2011-11-02 18:44, Fabien Chouteau wrote:
>>>> On 31/10/2011 14:12, Peter Maydell wrote:
>>>>> On 29 October 2011 14:52, Alexander Graf<agraf@suse.de>   wrote:
>>>>>> A lot of people seem to also have code that doesn't make sense
>>>>>> upstream, for example implementing a one-off device that only
>>>>>> really matters for their own devboard which nobody else owns.
>>>>>> For such cases, having a plugin framework would be handy. I
>>>>>> interestingly enough got into the same discussion on LinuxCon
>>>>>> with some QEMU downstreams.
>>>>>
>>>>> If we get the qdev rework done then I think we're probably in
>>>>> a better position to have a plugin framework for devices. (There
>>>>> are some issues about API and ABI stability guarantees, of course.)
>>>>>
>>>>
>>>> Interesting, we have a "plug-in" implementation in our Qemu branch. It
>>>
>>> We have a "plugin" model here as well. It's really simple: the plugin is
>>> loaded dynamically into the QEMU process and can access any global
>>> function and variable. Of course, this breaks regularly.
>>
>> Yes, this is the Right Model.
>>
>> All of the work is in making the interfaces not break regularly.
>> Loading a shared object is easy enough.
>
> I agree. In fact, we could even do it the same way as the kernel and
> build all our internal hw pieces as shared objects.
>
> Then users who want to cut down QEMU can just remove .so files instead
> of messing with the build system or code.

This is totally untested and won't probably build, but we can do something like 
this.  We should also follow 
http://www.gnu.org/prep/standards/html_node/Dynamic-Plug_002dIn-Interfaces.html

diff --git a/hw/qdev.c b/hw/qdev.c
index 50976dd..1d5aea7 100644
--- a/hw/qdev.c
+++ b/hw/qdev.c
@@ -58,6 +58,7 @@ static DeviceInfo *qdev_find_info(BusInfo *bus_info, const cha
  {
      DeviceInfo *info;

+again:
      /* first check device names */
      for (info = device_info_list; info != NULL; info = info->next) {
          if (bus_info && info->bus_info != bus_info)
@@ -77,6 +78,20 @@ static DeviceInfo *qdev_find_info(BusInfo *bus_info, const ch
              continue;
          return info;
      }
+
+    /* try to load an appropriately named module */
+    {
+        char *path = g_module_build_path(PREFIX, info->name);
+        GModule *mod = g_module_open(path, G_MODULE_BIND_LOCAL);
+        void (*init)(void);
+
+        if (g_module_symbol(mod, "mod_init", &init)) {
+            init();
+            goto again;
+        }
+    }
+
+
      return NULL;
  }

Regards,

Anthony Liguori

>
>
> Alex
>
>