Re: PowerPC: gettimeofday optimization by using IFUNC

[Richard Henderson <rth at twiddle dot net>]

On 03/14/2013 09:40 AM, Adhemerval Zanella wrote:
> Maybe if I rephrase the comments to:
> 
> /* This macros is need for PPC64 to return the ODP entry of vDSO symbol
>    since _dl_vdso_vsym always return the function address and the fixup
>    code expects a ODP entry.  */
> 
> What do you think?

You're still missing the fact that either:

(1) __vdso_gettimeofday is a pointer to an OPD and so taking the address of
__vdso_gettimeofday is wrong, or

(2) __vdso_gettimeofday is not itself an OPD.

I don't really believe (1) is true because that would never have survived even
minimal testing with that address-of operator included.

Recall that the format of an OPD is:

  struct {
    void *code_ptr;
    void *toc_ptr;
    void *chain_ptr;
  };

It's clear that "&pointer" does not qualify and thus (2) is true.


How about something like:

/* This macro is needed for PPC64 to return a skeleton OPD entry of a vDSO
   symbol.  This works because _dl_vdso_vsym always return the function
   address, and no vDSO symbols use the TOC or chain pointers from the OPD
   so we can allow them to be garbage.  */

and then name the macro "VDSO_IFUNC_RET" to make it clear we're not talking
about generic pointers as "PTR_IFUNC_RET" might suggest.


Future work might include cleaning all this up so that it's much more obvious
what's going on.  Consider:

struct __vdso_opd {
  void *code;
#ifndef USE_PPC64_OVERLAPPING_OPD
  void *toc;
  void *chain;
#endif
};

typedef struct __vdso_opd __vdso_ret;

Change the return type of _dl_vdso_vsym generically to __vdso_ret, where the
default version is a typedef to void*.

I'm not sure that you can adjust DL_SYMBOL_ADDRESS, as that's also used by the
regular part of the dynamic linker.  You could either invent a new macro to be
used by the generic dl-vdso.c, or make a private copy for ppc64.  If a new
macro, you could fairly easily compute the value as (__vdso_ret){ ... } to fill
in only the first entry of the structure (letting C rules fill the rest w/0).

Adjust VDSO_IFUNC_RET so that the type of the object passed is validated.


r~