Re: [Qemu-devel] [PATCH 10/19] Add a query-machines command to QMP

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Jun 07, 2010 at 10:13:27AM -0500, Anthony Liguori wrote:
> On 06/07/2010 09:42 AM, Daniel P. Berrange wrote:
> >Add a new QMP monitor command 'query-machines' to discover what
> >machines are defined in the QEMU binary. This is an easily
> >parsable replacement for 'qemu -M ?'
> >
> >     [
> >         {
> >             "name": "pc-0.13",
> >             "description": "Standard PC",
> >             "default": 0
> >         },
> >         {
> >             "name": "pc",
> >             "description": "Standard PC",
> >             "canonical": "pc-0.13",
> >             "default": 1
> >         },
> >         {
> >             "name": "pc-0.12",
> >             "description": "Standard PC",
> >             "default": 0
> >         },
> >         ....
> >     ]
> >
> >No legacy readline monitor output is provided.
> >
> >In the future it would be desirable for each machine's QDict to
> >also include details of any qdev devices that are included by
> >default in the machine type.
> >
> >Signed-off-by: Daniel P. Berrange<berrange@redhat.com>
> >---
> >  hw/boards.h |    1 +
> >  monitor.c   |    9 +++++++
> >  vl.c        |   70 
> >  +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 80 insertions(+), 0 deletions(-)
> >
> >diff --git a/hw/boards.h b/hw/boards.h
> >index 6f0f0d7..2f6003d 100644
> >--- a/hw/boards.h
> >+++ b/hw/boards.h
> >@@ -32,6 +32,7 @@ typedef struct QEMUMachine {
> >  } QEMUMachine;
> >
> >  int qemu_register_machine(QEMUMachine *m);
> >+void do_info_machines(Monitor *mon, QObject **data);
> >
> >  extern QEMUMachine *current_machine;
> >
> >diff --git a/monitor.c b/monitor.c
> >index f0406e8..b6aa2b4 100644
> >--- a/monitor.c
> >+++ b/monitor.c
> >@@ -55,6 +55,7 @@
> >  #include "json-streamer.h"
> >  #include "json-parser.h"
> >  #include "osdep.h"
> >+#include "hw/boards.h"
> >
> >  //#define DEBUG
> >  //#define DEBUG_COMPLETION
> >@@ -2449,6 +2450,14 @@ static const mon_cmd_t info_cmds[] = {
> >          .mhandler.info_new = do_info_version,
> >      },
> >      {
> >+        .name       = "machines",
> >+        .args_type  = "",
> >+        .params     = "",
> >+        .help       = "show the machine boards",
> >+        .user_print = monitor_user_noop,
> >+        .mhandler.info_new = do_info_machines,
> >+    },
> >+    {
> >          .name       = "commands",
> >          .args_type  = "",
> >          .params     = "",
> >diff --git a/vl.c b/vl.c
> >index 0b38d62..8043fac 100644
> >--- a/vl.c
> >+++ b/vl.c
> >@@ -1537,6 +1537,76 @@ static QEMUMachine *find_default_machine(void)
> >      return NULL;
> >  }
> >
> >+
> >+/**
> >+ * do_info_machines(): Show machine boards
> >+ *
> >+ * Returns a QList object listing all machine boards
> >+ * available with this QEMU target. Each element in
> >+ * the list is a QDict object containing the following
> >+ * keys
> >+ *
> >+ *  - "name": short name for the machine board
> >+ *  - "description": long description of the board
> >+ *  - "alias": name of an alias
> >+ *
> >+ * Example:
> >+ *
> >+ *  [
> >+ *     {
> >+ *       "name": "pc-0.13",
> >+ *        "description": "Standard PC",
> >+ *        "default": 0
> >+ *     },
> >+ *     {
> >+ *       "name": "pc",
> >+ *       "description": "Standard PC",
> >+ *       "canonical": "pc-0.13",
> >+ *       "default": 1
> >+ *     },
> >+ *     {
> >+ *       "name": "pc-0.12",
> >+ *       "description": "Standard PC",
> >+ *       "default": 0
> >+ *     },
> >+ *     ....
> >+ *  ]
> >+ *
> >+ */
> >   
> 
> My only suggestion would be to:
> 
> { 'default-machine': 'pc',
>    'machines': [{
>      'name': 'pc',
>      'description': 'Standard PC',
>    },...]
> }
> 
> I'd drop 'canonical' too.  Aliasing is an implementation detail and 
> should not be exposed via the ABI.

The alias/canonical mapping is something that libvirt needs to know in
order guarentee stable guest ABI for all configs it has.

What happens is that an app using libvirt will request a guest config
with machine type 'pc', and libvirt resolves that to the canonical
type 'pc-0.12', and then launches QEMU with '-M pc-0.12' instead of
the '-M pc'. If libvirt passed '-M pc' and let QEMU do the canonicalization,
the guest ABI would no longer be stable across QEMU upgrades because the
canonicalization changes to point to the newer version.

Regards,
Daniel
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