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Re: [Qemu-devel] [PATCH v10 5/8] module: implement module loading

From: Fam Zheng
Subject: Re: [Qemu-devel] [PATCH v10 5/8] module: implement module loading
Date: Mon, 16 Sep 2013 19:29:17 +0800
User-agent: Mutt/1.5.21 (2010-09-15)

On Mon, 09/16 12:30, Paolo Bonzini wrote:
> Il 16/09/2013 12:21, Daniel P. Berrange ha scritto:
> > On Mon, Sep 16, 2013 at 12:18:54PM +0200, Paolo Bonzini wrote:
> >> Il 16/09/2013 12:14, Daniel P. Berrange ha scritto:
> >>> On Mon, Sep 16, 2013 at 12:09:47PM +0200, Paolo Bonzini wrote:
> >>>> Il 16/09/2013 11:51, Fam Zheng ha scritto:
> >>>>> On Mon, 09/16 11:44, Paolo Bonzini wrote:
> >>>>>> Il 16/09/2013 10:59, Daniel P. Berrange ha scritto:
> >>>>>>>> The init function of dynamic module is no longer with
> >>>>>>>> __attribute__((constructor)) as static linked version, and need to be
> >>>>>>>> explicitly called once loaded. The function name is mangled with per
> >>>>>>>> configure fingerprint as:
> >>>>>>>>
> >>>>>>>>     init_$(date +%s$$$RANDOM)
> >>>>>>
> >>>>>> Does this work for a module that calls module_init multiple times?
> >>>>>
> >>>>> Why should a module calls module_init, instead of the main function?
> >>>>
> >>>> I think you mean "why should a module calls register_module_init", and I
> >>>> agree that with this patch a module will not call register_module_init.
> >>>>
> >>>> But a module is still using the module_init macro.
> >>>>
> >>>> With this patch, a module will not be able to use the module_init macro
> >>>> twice.  I am not sure this is an acceptable limitation, especially if we
> >>>> do not have a dependency system within modules and/or load them with
> >>>
> >>> Why would a module ever want to use the module_init macro twice ?
> >>
> >> Because our coding standard is to have each source file do its own
> >> one-time initialization, using static functions and an invocation of
> >> module_init per source file.
> > 
> > Is there ever a case where two source files, each using module_init
> > will be compiled into the same .so loadable module. Looking at the
> > uses of block_init(), I don't see any obvious candidates for trouble,
> > all uses look like they'd be going into separate .so files.
> Without inter-module exports, all of SPICE probably would have to be in
> a single .so file.  This includes spice-qemu-char.c and
> hw/display/qxl.c, both of which use type_init.
> If we use G_MODULE_GLOBAL as a primitive system for intermodule exports,
> then indeed this is a much smaller problem, but then we need a
> dependency system.  But I'm almost sure that Windows and maybe Darwin
> lack support for the equivalent of G_MODULE_GLOBAL.

An idea for single .so file:
    - before loads a .so, an empty initializer list is created.
    - module_init adds a __attribute__((constructor)) function, which appends
      its real initializer to the initializer list. So this function is
      automatically called after dlopen().
    - make init_$(date +%s$$$RANDOM) a dummy symbol.
    - module_load first checks the presense of the symbol, if yes, call the
      functions in the initializer list. Else clean up and unload .so.

Does this enable multiple calls of module_init()?

OTOH. As for multiple spice modules, is it possible to solve it by having a
spice-common.o and link all spice modules to it, to share code?


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