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

[Michael Roth <mdroth@linux.vnet.ibm.com>]

On 01/12/2012 12:57 PM, Luiz Capitulino wrote:
> 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.
>

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);
>