Re: [RFC PATCH v2 02/10] lib: vdso: move call to fallback out of common code.

[Andy Lutomirski <luto@kernel.org>]

On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy
<christophe.leroy@c-s.fr> wrote:
>
> On powerpc, VDSO functions and syscalls cannot be implemented in C
> because the Linux kernel ABI requires that CR[SO] bit is set in case
> of error and cleared when no error.
>
> As this cannot be done in C, C VDSO functions and syscall'based
> fallback need a trampoline in ASM.
>
> By moving the fallback calls out of the common code, arches like
> powerpc can implement both the call to C VDSO and the fallback call
> in a single trampoline function.

Maybe the issue is that I'm not a powerpc person, but I don't
understand this.  The common vDSO code is in C.  Presumably this means
that you need an asm trampoline no matter what to call the C code.  Is
the improvement that, with this change, you can have the asm
trampoline do a single branch, so it's logically:

ret = [call the C code];
if (ret == 0) {
 set success bit;
} else {
 ret = fallback;
 if (ret == 0)
  set success bit;
else
  set failure bit;
}

return ret;

instead of:

ret = [call the C code, which includes the fallback];
if (ret == 0)
  set success bit;
else
  set failure bit;

It's not obvious to me that the former ought to be faster.

>
> The two advantages are:
> - No need play back and forth with CR[SO] and negative return value.
> - No stack frame is required in VDSO C functions for the fallbacks.

How is no stack frame required?  Do you mean that the presence of the
fallback causes worse code generation?  Can you improve the fallback
instead?