Re: [PATCH v17 14/18] trace-cmd: Add host trace clock as guest trace argument

[Steven Rostedt <rostedt@goodmis.org>]

On Tue, 10 Dec 2019 10:49:43 +0200
Tzvetomir Stoyanov <tz.stoyanov@gmail.com> wrote:

> > > +     if (guest_config) {
> > > +             /* If -C is specified, prepend clock to all guest VM flags */
> > > +             for_all_instances(instance) {
> > > +                     if (top_instance.ftrace->clock) {
> > > +                             if (is_guest(instance)) {  
> >
> > We should only append this, if the guest didn't have a clock set
> > already. As the change log seems to say, if the user states a "-C
> > clock" for the guest, that should take precedence over the host clock
> > set. That is, a user may specifically state that they are using a
> > different clock. If we have frequency and offset set, it should still
> > work with different clocks.  
> 
> The guest config string is not parsed in the host context, that's why the host
> doesn't know if a guest has an explicit  "-C clock" argument.
> I can parse the guest config here, but this will complicate the implementation.
> Using the current approach still guarantees that the user specified
> config has higher
> priority than injected one - add_argv() API prepends to the beginning
> of the string, so
> user arguments are always after the injected one. When guest parses
> the string, in case of
> duplicated "-C clock" arguments, the last one wins.

I'm confused. I'm looking at this:


	for (;;) {
		[..]
		switch (c) {
		[..]
		case 'A':
			[..]
			ctx->instance->flags |= BUFFER_FL_GUEST;
		[..]
		case 'C':
			ctx->instance->ftrace->clock = optarg;
			guest_config = true;
			break;
		[..]
		}
		[..]
	}
	[..]
	if (guest_config) {
		/* If -C is specified, prepend clock to all guest VM flags */
		for_all_instances(instance) {
			if (top_instance.ftrace->clock) {

Why can't we have here:

 if (top_instance.ftrace->clock && !instance->ftrace->clock)

If the guest instance was given a -C, I would think we don't want to add
another -C to pass to that guest?

-- Steve

				if (is_guest(instance)) {
					add_argv(instance,
						 (char *)top_instance.ftrace->clock,
						 true);
					add_argv(instance, "-C", true);
				}
			}
		}
	}