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

[Michael Roth]

On 01/12/2012 12:57 PM, Luiz Capitulino wrote:
> On Wed, 11 Jan 2012 17:56:05 -0600
> Michael Roth<address@hidden>  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<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' } }
>
> 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.

Agreed, I'll precede this with a patch to add the documentation for 
existing types, and update this patch accordingly.

> >   { '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.

I must've looked at this like 5 times and made a mental note to do that, 
but in the end it escaped me...

> > +#
> > +# 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).

The _MIN vs. _DEFAULT is there because we may some time in the future 
decide to bump up the default support level. We'd still allow fallback 
to _MIN, if needed, but it may begin to impede progress in the future. 
But yah, technically since *_MIN == *_DEFAULT I could get rid of them 
till we need them, I just wanted to make it clear in the code from the 
start. Same with making the call...ga_set_support_level() may at some 
point take list of capabilities or something...just wanted to make a 
central point where all the initialization happens. But I don't mind 
losing it if that's too much ugly-now-but-might-make-sense-later stuff.

> >       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);