Re: [Qemu-devel] Thoughts around dtrace linking...

[Stefan Hajnoczi]

On Wed, Mar 21, 2012 at 1:01 PM, Andreas Färber <address@hidden> wrote:
> Hi,
>
> Am 21.03.2012 11:45, schrieb Lee Essen:
>> I've been trying to find a sensible way to solve the Solaris/Illumos
>> dtrace requirement to pass all the objs to the dtrace command so that
>> the resultant object file contains all the symbols needed to properly
>> link the relevant binary.
>>
>> The easiest way to do this is just prior to linking the binary, so
>> something like this (in rules.mak):
>>
>>     LINK = $(call quiet-command,$(CC) $(QEMU_CFLAGS) $(CFLAGS)
>>     $(LDFLAGS) -o $@ $(sort $(1)) $(LIBS),"  LINK  $(TARGET_DIR)$@")
>>
>>     DTRACE = $(call quiet-command,dtrace $(CONFIG_DTRACE_FLAGS) -o
>>     $(1)-dtrace.o -G -s $(2) $(3), "  GEN $(TARGET_DIR)$(1)-dtrace.o")
>>
>>     %$(EXESUF): %.o
>>       $(call DTRACE,$*,trace-dtrace.dtrace,$^)
>>       $(call LINK,$^ $*-dtrace.o)

What I find slightly surprising is that you're putting the -dtrace.o
generation step as a command in the executable's target.

I would expect the -dtrace.o to be a target itself, which also allows
make to use it's timestamping on dependencies to ensure we only
rebuild when necessary.  i.e. specifying dependencies is the make way
of doing things, and I think we should try where possible.

>>
>> Obviously with the relevant tests around it to check the trace backend,
>> and also an adjustment in Makefile.target to cause the right thing to
>> happen for each target.
>> Or, is there a better way?
>
> The two issues I see (as info for Stefan et al.) are
> a) compiling DTrace probes into .o files requires linking those objects
> with that additional .o file to avoid linker errors (even for tools
> where using DTrace probes does not seem to make much sense),

qemu-tool binaries are built with tracing enabled.  But this is a good
point, we need to check that all binaries buildable from the QEMU
source tree continue to work with this change.

Stefan