Re: [Qemu-devel] [PATCH] Add CONFIG_QEMU_TIMER to handle qemu-timer-common.o dep

[Anthony Liguori]

On 08/30/2011 04:22 AM, Stefan Hajnoczi wrote:
> On Mon, Aug 29, 2011 at 8:27 PM, Michael Roth<address@hidden>  wrote:
> > @@ -380,7 +381,6 @@ else
> >   trace-obj-y = trace.o
> >   ifeq ($(TRACE_BACKEND),simple)
> >   trace-obj-y += simpletrace.o
> > -user-obj-y += qemu-timer-common.o
> >   endif
> >   endif
>
> Now that we have a concrete patch to look at I think this approach is
> problematic.  There are several subsystems in QEMU which might be
> built outside the main qemu binary for qemu-io, qemu-img, qemu-ga,
> etc.

Er, but qemu-timer cannot possibly be used by qemu-io/qemu-img.

Is this all dummy magic in order to let qemu-io build even though simple 
tracing won't work?

Perhaps we should look at making the tracing backends dynamic instead of 
static?

Regards,

Anthony Liguori

> Each subsystem should explicitly include its dependencies (e.g.
> subsys-obj-y += qemu-timer-common.o or subsys-obj-y +=
> $(more-fundamental-subsys)) so that it can be easily used by a target.
>   If this is not done then there are two disadvantages:
> 1. We spray dependency information across the makefiles instead of
> keeping them contained with the subsystem that has the dependency
> requirement.
> 2. We duplicate the dependencies across each target in the form of
> conditional objects:
> x-obj-$(CONFIG_MY_DEPENDENCY) += ...
>
> If QEMU is split up into libraries then having an explicit list of
> dependencies for each subsystem will be very useful, whereas the
> CONFIG_* approach doesn't collect that information in one place.
>
> So I think explicit subsys-obj-y += qemu-timer-common.o together with
> $(sort) during the link stage actually allows for a cleaner build
> system.  I prefer that approach.
>
> Stefan