[Qemu-devel] [PATCH] qemu-ga: add guest-set-support-level command

[Michael Roth]

Recently commands where introduced on the mailing that involved adding
commands to the guest agent that could potentially break older versions
of QEMU. While it's okay to expect that qemu-ga can be updated to support
newer host features, it's unrealistic to require a host to be updated to
support qemu-ga features, or to expect that qemu-ga should be downgraded
in these circumstances, especially considering that a large mix of
guests/agents may be running on a particular host.

To address this, we introduce here a mechanism to set qemu-ga's
feature-set to one that is known to be compatible with
the host/QEMU running the guest. As new commands/options are added, we
can maintain backward-compatibility by adding conditional checks to filter
out host-incompatible functionality based on the host-specified support
level (generally analogous to the host QEMU version) set by the
client.

The current default/minimum support level supports all versions of QEMU
that have had qemu-ga in-tree (0.15.0, 1.0.0) and so should be
backward-compatible with existing hosts/clients.

Signed-off-by: Michael Roth <address@hidden>
---
 qapi-schema-guest.json     |   31 ++++++++++++++++++++++++++++-
 qemu-ga.c                  |   46 ++++++++++++++++++++++++++++++++++++++++++++
 qga/guest-agent-commands.c |   13 ++++++++++++
 qga/guest-agent-core.h     |   11 ++++++++++
 4 files changed, 100 insertions(+), 1 deletions(-)

diff --git a/qapi-schema-guest.json b/qapi-schema-guest.json
index 5f8a18d..32bc041 100644
--- a/qapi-schema-guest.json
+++ b/qapi-schema-guest.json
@@ -43,15 +43,44 @@
 #
 # Since: 0.15.0
 ##
+{ 'type': 'GuestAgentSupportLevel',
+  'data': { 'major': 'int', 'minor': 'int', 'micro': 'int' } }
 { 'type': 'GuestAgentCommandInfo',
   'data': { 'name': 'str', 'enabled': 'bool' } }
 { 'type': 'GuestAgentInfo',
   'data': { 'version': 'str',
-            'supported_commands': ['GuestAgentCommandInfo'] } }
+            'supported_commands': ['GuestAgentCommandInfo']
+            'support_level': 'GuestAgentSupportLevel' } }
 { 'command': 'guest-info',
   'returns': 'GuestAgentInfo' }
 
 ##
+# @guest-set-support-level:
+#
+# Set guest agent feature-set to one that is compatible with/supported by
+# the host.
+#
+# Certain commands/options may have dependencies on host
+# version/support-level, such as specific QEMU features (such
+# dependencies will be noted in this schema). By default we assume 1.0.0,
+# which is backward-compatible with QEMU 0.15.0/1.0, and should be compatible
+# with later versions of QEMU as well. To enable newer guest agent features,
+# this command must be issued to raise the support-level to one corresponding
+# to supported host QEMU/KVM/etc capabilities.
+#
+# The currently set support level is obtainable via the guest-info command.
+#
+# @level: Desired host support-level, generally <= host QEMU version
+# level. Note that undefined behavior may occur if a support-level is
+# provided that exceeds the capabilities of the version of QEMU currently
+# running the guest.
+#
+# Returns: Nothing on success
+#
+{ 'command': 'guest-set-support-level',
+  'data':    { 'major': 'int', 'minor': 'int', '*micro': 'int' } }
+
+##
 # @guest-shutdown:
 #
 # Initiate guest-activated shutdown. Note: this is an asynchronous
diff --git a/qemu-ga.c b/qemu-ga.c
index 200bb15..3b3175c 100644
--- a/qemu-ga.c
+++ b/qemu-ga.c
@@ -28,6 +28,7 @@
 #include "qerror.h"
 #include "error_int.h"
 #include "qapi/qmp-core.h"
+#include "qga-qapi-types.h"
 
 #define QGA_VIRTIO_PATH_DEFAULT "/dev/virtio-ports/org.qemu.guest_agent.0"
 #define QGA_PIDFILE_DEFAULT "/var/run/qemu-ga.pid"
@@ -102,6 +103,45 @@ static void usage(const char *cmd)
 
 static void conn_channel_close(GAState *s);
 
+static GuestAgentSupportLevel ga_support_level;
+
+static int ga_cmp_support_levels(GuestAgentSupportLevel a,
+                                 GuestAgentSupportLevel b)
+{
+    if (a.major == b.major) {
+        if (a.minor == b.minor) {
+            return a.micro - b.micro;
+        }
+        return a.minor - b.minor;
+    }
+    return a.major - b.major;
+}
+
+bool ga_has_support_level(int major, int minor, int micro)
+{
+    GuestAgentSupportLevel level = { major, minor, micro };
+    return ga_cmp_support_levels(level, ga_support_level);
+}
+
+void ga_set_support_level(GuestAgentSupportLevel level)
+{
+    GuestAgentSupportLevel min_support_level = {
+        QGA_SUPPORT_LEVEL_MAJOR_MIN,
+        QGA_SUPPORT_LEVEL_MINOR_MIN,
+        QGA_SUPPORT_LEVEL_MICRO_MIN
+    };
+    if (ga_cmp_support_levels(level, min_support_level) <= 0) {
+        ga_support_level = min_support_level;
+    } else {
+        ga_support_level = level;
+    }
+}
+
+GuestAgentSupportLevel ga_get_support_level(void)
+{
+    return ga_support_level;
+}
+
 static const char *ga_log_level_str(GLogLevelFlags level)
 {
     switch (level & G_LOG_LEVEL_MASK) {
@@ -569,6 +609,11 @@ int main(int argc, char **argv)
     GLogLevelFlags log_level = G_LOG_LEVEL_ERROR | G_LOG_LEVEL_CRITICAL;
     FILE *log_file = stderr;
     GAState *s;
+    GuestAgentSupportLevel default_support_level = {
+        QGA_SUPPORT_LEVEL_MAJOR_DEFAULT,
+        QGA_SUPPORT_LEVEL_MINOR_DEFAULT,
+        QGA_SUPPORT_LEVEL_MICRO_DEFAULT
+    };
 
     module_call_init(MODULE_INIT_QAPI);
 
@@ -642,6 +687,7 @@ int main(int argc, char **argv)
         become_daemon(pidfile);
     }
 
+    ga_set_support_level(default_support_level);
     s = g_malloc0(sizeof(GAState));
     s->conn_channel = NULL;
     s->path = path;
diff --git a/qga/guest-agent-commands.c b/qga/guest-agent-commands.c
index a09c8ca..656dde8 100644
--- a/qga/guest-agent-commands.c
+++ b/qga/guest-agent-commands.c
@@ -62,6 +62,8 @@ struct GuestAgentInfo *qmp_guest_info(Error **err)
     char **cmd_list_head, **cmd_list;
 
     info->version = g_strdup(QGA_VERSION);
+    info->support_level = g_malloc0(sizeof(GuestAgentSupportLevel));
+    *info->support_level = ga_get_support_level();
 
     cmd_list_head = cmd_list = qmp_get_command_list();
     if (*cmd_list_head == NULL) {
@@ -87,6 +89,17 @@ out:
     return info;
 }
 
+void qmp_guest_set_support_level(int64_t major, int64_t minor, bool has_micro,
+                                 int64_t micro, Error **errp)
+{
+    GuestAgentSupportLevel level = {
+        major,
+        minor,
+        has_micro ? micro : 0
+    };
+    ga_set_support_level(level);
+}
+
 void qmp_guest_shutdown(bool has_mode, const char *mode, Error **err)
 {
     int ret;
diff --git a/qga/guest-agent-core.h b/qga/guest-agent-core.h
index e42b91d..7327e73 100644
--- a/qga/guest-agent-core.h
+++ b/qga/guest-agent-core.h
@@ -12,8 +12,16 @@
  */
 #include "qapi/qmp-core.h"
 #include "qemu-common.h"
+#include "qga-qapi-types.h"
 
 #define QGA_VERSION "1.0"
+#define QGA_SUPPORT_LEVEL_MAJOR_DEFAULT 1
+#define QGA_SUPPORT_LEVEL_MINOR_DEFAULT 0
+#define QGA_SUPPORT_LEVEL_MICRO_DEFAULT 0
+/* lowest possible support level */
+#define QGA_SUPPORT_LEVEL_MAJOR_MIN 1
+#define QGA_SUPPORT_LEVEL_MINOR_MIN 0
+#define QGA_SUPPORT_LEVEL_MICRO_MIN 0
 #define QGA_READ_COUNT_DEFAULT 4 << 10
 
 typedef struct GAState GAState;
@@ -29,3 +37,6 @@ GACommandState *ga_command_state_new(void);
 bool ga_logging_enabled(GAState *s);
 void ga_disable_logging(GAState *s);
 void ga_enable_logging(GAState *s);
+bool ga_has_support_level(int major, int minor, int micro);
+void ga_set_support_level(GuestAgentSupportLevel level);
+GuestAgentSupportLevel ga_get_support_level(void);
-- 
1.7.4.1