Re: [RFC PATCH v2 04/10] lib: vdso: get pointer to vdso data from the arch

[Andy Lutomirski]

On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy
<christophe.leroy@xxxxxx> wrote:
>
> On powerpc, __arch_get_vdso_data() clobbers the link register,
> requiring the caller to set a stack frame in order to save it.
>
> As the parent function already has to set a stack frame and save
> the link register to call the C vdso function, retriving the
> vdso data pointer there is lighter.

I'm confused.  Can't you inline __arch_get_vdso_data()?  Or is the
issue that you can't retrieve the program counter on power without
clobbering the link register?

I would imagine that this patch generates worse code on any
architecture with PC-relative addressing modes (which includes at
least x86_64, and I would guess includes most modern architectures).

--Andy