Re: [PATCH 5/5 V2] kvm tools: Initialize and use VESA and VNC

[Paolo Bonzini <pbonzini@redhat.com>]

On 05/24/2011 09:40 PM, Ingo Molnar wrote:
> >  Ah, forgot about this.  Given_why_  this happens (for static libraries, the
> >  linker omits object modules that are not required to fulfill undefined
> >  references in previous objects), I'd be surprised if explicit section tricks
> >  do not have the same limitation.  [...]
>
> Since we create an actual array (data)

Are you?  I figured it was like this example from Linux:

         .x86_cpu_dev.init : AT(ADDR(.x86_cpu_dev.init) - LOAD_OFFSET) {
                 __x86_cpu_dev_start = .;
                 *(.x86_cpu_dev.init)
                 __x86_cpu_dev_end = .;
         }


   extern const struct cpu_dev *const __x86_cpu_dev_start[],
                               *const __x86_cpu_dev_end[];

   for (cdev = __x86_cpu_dev_start; cdev < __x86_cpu_dev_end; cdev++) {

This is pretty much the same as the linker script for ELF:

   .init_array     : {
      __init_array_start = .;
      *(.init_array)
      *(SORT(.init_array$*))
      __init_array_end = .;
   }

   extern void (*__init_array_start []) (int, char **, char **)
     attribute_hidden;
   extern void (*__init_array_end []) (int, char **, char **)
     attribute_hidden;

   const size_t size = __init_array_end - __init_array_start;
   for (size_t i = 0; i < size; i++)
       (*__init_array_start [i]) (argc, argv, envp);


> ((constructor)) has showstopper properties:
>
>  - We don't have access to the program arguments
>
>  - stdio is probably not set up yet (this is undefined AFAICS)

As I said, you can do this even better by doing only minimal work in the 
constructor.  Create a struct with several callbacks (pre_init, 
late_init, destroy, reset, whatever) and possibly other information (a 
human-readable device name and command-line argument to access it, for 
example).  In the constructor you just build a linked list of said 
structs, and then you can walk it whenever you see fit: do comparisons, 
call a function, whatever.  This is similar to the cpudev example from 
the kernel above.

A simple example from QEMU:

   static void virtio_pci_register_devices(void)
   {
       pci_qdev_register_many(virtio_info);
   }

   /* This macro wraps ((constructor)).  */
   device_init(virtio_pci_register_devices)

>    In that sense ((section)) is way more robust: there's not really that many
>    ways to screw that up. Fiddling with the ((constructor)) environment on the
>    other hand ...

Sorry, this is textbook FUD.

> __attribute__((constructor)) is not particularly portable to begin with: does
> the MSVC compiler support it for example?

No, but GCC supports it on non-ELF platforms, where you would need a 
similar but different linker script.  (Also, the differences between 
MSVC and GCC can easily be abstracted with a macro).

More practically, is your linker script supported by gold?

Paolo