Re: [Qemu-devel] [PATCH] Extend qemu-ga's 'guest-info' command to expose flag 'success-response'

[Luiz Capitulino <lcapitulino@redhat.com>]

On Tue, 24 Sep 2013 16:07:29 -0500
Michael Roth <mdroth@linux.vnet.ibm.com> wrote:

> > >> +bool qmp_command_has_success_response(const char *name)
> > >> +{
> > >> +    QmpCommand *cmd;
> > >> +
> > >> +    QTAILQ_FOREACH(cmd, &qmp_commands, node) {
> > >> +        if (strcmp(cmd->name, name) == 0) {
> > >> +            return cmd->options != QCO_NO_SUCCESS_RESP;
> > 
> > cmd->options is a bitmask - it is feasible that we may add more QCO_NO_*
> > flags in the future, at which point inequality is NOT correct.  Rather,
> > you want:
> > 
> > return !(cmd->options & QCO_NO_SUCCESS_RESP);

Good catch! IIRC I added cmd->options myself and didn't catch this...

> > >> +++ b/qga/commands.c
> > >> @@ -63,6 +63,8 @@ struct GuestAgentInfo *qmp_guest_info(Error **err)
> > >>          cmd_info = g_malloc0(sizeof(GuestAgentCommandInfo));
> > >>          cmd_info->name = g_strdup(*cmd_list);
> > >>          cmd_info->enabled = qmp_command_is_enabled(cmd_info->name);
> > >> +        cmd_info->success_response =
> > >> +            qmp_command_has_success_response(cmd_info->name);
> > 
> > This feels wasteful.  Why are we doing an O(n) lookup for BOTH
> > qmp_command_is_enabled AND qmp_command_has_success_response, in an O(n)
> > loop over command names?  That's O(n^2) in the number of commands.
> > Better would be getting a list of QmpCommand* instead of a list of
> > char*, and looking directly in each object, for O(n) computation of the
> > results.
> 
> Agreed, modifying qmp_get_command_list to return a list of QmpCommand
> would be nicer. Rather than looking directly at the fields though I
> think we should just fix up qmp_command_is_enabled() and friends to
> take a QmpCommand arg instead of a char*. We already have
> qmp_find_command to map char*->QmpCommand to support any cases where
> we rely on cmd names.

I agree and I thought the same thing when I reviewed the patch, but
I didn't mind as Mark is just using what's already there.