Hello,

The attached patch adds the ability to specify a wildcard machine when 
registering an OSABI / arch / machine.  It then updates PPC64 GNU/Linux 
to specify that wild card (-1) instead of zero as the default machine.

Looking at the PPC64 GNU/Linux code:

   gdbarch_register_osabi (bfd_arch_powerpc, 0, GDB_OSABI_LINUX,
                           ppc_linux_init_abi);

I believe that the call is attempting to register ppc_linux_init_abi as 
the OSABI handler for all arch/machine conbinations.  The problem is 
that machine "0" gets turned into bfd_mach_ppc or bfd_mach_ppc64 
dependant on how GDB is built, and they are both incompatible with each 
other and incompatible .  And that in turn restricts the support to just 
one half of the ISA family making it impossible for GDB to debug both 32 
and 64 bit :-(

I know of two ways to fix this.  First is the attached patch which 
modifies osabi.[hc] so that a wildcard machine (-1) can be specified vis:

   gdbarch_register_osabi (bfd_arch_powerpc, -1, GDB_OSABI_LINUX,
                           ppc_linux_init_abi);

and the second is to explicitly register both of these architecture 
variants vis:

   gdbarch_register_osabi (bfd_arch_powerpc, bfd_mach_ppc, ...
   gdbarch_register_osabi (bfd_arch_powerpc, bfd_mach_ppc64, ...

(possibly also splitting ppc_linux_init_abi into ppc_linux_init_abi_32 
and ppc_linux_init_abi_32).

There are pros and cons to both.

The former will always match, making the code somewhat future proof, the 
later is far simplier.

preferences?
Andrew

PS: I'm also wondering if the existing ppc arch/machine table is 
complete.  I think it will indicate that e500 is "compatible" with "ppc" 
when it is not.