> On Dec 24, 2019, at 7:53 PM, christophe leroy <christophe.le...@c-s.fr> wrote:
>
>
>
>> Le 24/12/2019 à 03:27, Andy Lutomirski a écrit :
>>> On Mon, Dec 23, 2019 at 6:31 AM Christophe Leroy
>>> <christophe.le...@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.
I mean PC-relative access for data. The data page is at a constant, known
offset from the vDSO text.
I haven’t checked how much x86_64 benefits from this, but at least the
non-array fields ought to be accessible with a PC-relative access.
It should be possible to refactor a little bit so that the compiler can still
see what’s going on. Maybe your patch actually does this. I’d want to look at
the assembly. This also might not matter much on x86_64 in particular, since
x86_64 can convert a PC-relative address to an absolute address with a single
instruction with no clobbers.
Does power have PC-relative data access? If so, I wonder if the code can be
arranged so that even the array accesses don’t require computing an absolute
address at any point.
