Re: [Qemu-devel] [PATCH v2] qemu-ga: add guest-set-support-level command

[Luiz Capitulino <lcapitulino@redhat.com>]

On Wed, 11 Jan 2012 17:56:05 -0600
Michael Roth <mdroth@linux.vnet.ibm.com> wrote:

> 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.

The approach looks fine to me. I have a few review comments below.

> 
> Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
> ---
>  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' } }

This and GuestAgentCommandInfo are missing good documentation. Looks like
we don't document types as we do in qapi-schema.json. I think we should.

>  { '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' }

For example, something important that's is not totally clear to me just by
reading the command definition is if 'support_level' refers to the support
level that can be changed by a client.

>  
>  ##
> +# @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.

It's also better to note that if @level < 1.0.0 then the support level will
be set to 1.0.0.

> +#
> +# 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..6840233 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) >= 0;
> +}
> +
> +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
> +    };

Can be const.

> +    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);

You could avoid that call, default_support_level and all the _DEFAULT
macros by initializing ga_support_level statically (using the _MIN macros).

>      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);