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

[christophe leroy <christophe.leroy@c-s.fr>]

Le 24/12/2019 à 03:27, Andy Lutomirski a écrit :
> On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy
> <christophe.leroy@c-s.fr> 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?

Yes it can be inlined (I did it in V1 
https://patchwork.ozlabs.org/patch/1180571/), but you can't do it 
without clobbering the link register, because the only way to get the 
program counter is to do to as if you were calling another function but 
you call to the address which just follows where you are, so that it 
sets LR which the simulated return address which corresponds to the 
address following the branch.

static __always_inline
const struct vdso_data *__arch_get_vdso_data(void)
{
	void *ptr;

	asm volatile(
		"	bcl	20, 31, .+4;\n"
		"	mflr	%0;\n"
		"	addi	%0, %0, __kernel_datapage_offset - (.-4);\n"
		: "=b"(ptr) : : "lr");

	return ptr + *(unsigned long *)ptr;
}

> 
> 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).

Why ? Powerpc is also using PC-relative addressing for all calls but 
indirect calls.

As the other arch C VDSO callers are in C and written in such a way that 
callee is inlined into callers, and as __arch_get_vdso_data() is 
inlined, it should make no difference, shouldn't it ?

Christophe