Re: [PATCH v7 04/25] arm64: Substitute gettimeofday with C implementation

[Vincenzo Frascino]

Hi Dave,

Overall, I want to thank you for bringing out the topic. It helped me to
question some decisions and make sure that we have no holes left in the approach.

[...]

>>
>> vDSO library is a shared object not compiled with LTO as far as I can
>> see, hence if this involved LTO should not applicable in this case.
> 
> That turned to be a spurious hypothesis on my part -- LTO isn't the
> smoking gun.  (See below.)
>

Ok.

>>> The classic example of this (triggered directly and not due to inlining)
>>> would be something like:
>>>
>>> int bar(int, int);
>>>
>>> void foo(int x, int y)
>>> {
>>> 	register int x_ asm("r0") = x;
>>> 	register int y_ asm("r1") = bar(x, y);
>>>
>>> 	asm volatile (
>>> 		"svc	#0"
>>> 		:: "r" (x_), "r" (y_)
>>> 		: "memory"
>>> 	);
>>> }
>>>
>>> ->
>>>
>>> 0000000000000000 <foo>:
>>>    0:   a9bf7bfd        stp     x29, x30, [sp, #-16]!
>>>    4:   910003fd        mov     x29, sp
>>>    8:   94000000        bl      0 <bar>
>>>    c:   2a0003e1        mov     w1, w0
>>>   10:   d4000001        svc     #0x0
>>>   14:   a8c17bfd        ldp     x29, x30, [sp], #16
>>>   18:   d65f03c0        ret
>>>
>>
>> Contextualized to what my vdso fallback functions do, this should not be a
>> concern because in no case a function result is directly set to a variable
>> declared as register.
>>
>> Since the vdso fallback functions serve a very specific and limited purpose, I
>> do not expect that that code is going to change much in future.
>>
>> The only thing that can happen is something similar to what I wrote in my
>> example, which as I empirically proved does not trigger the problematic behavior.
>>
>>>
>>> The gcc documentation is vague and ambiguous about precisely whan this
>>> can happen and about how to avoid it.
>>>
>>
>> On this I agree, it is not very clear, but this seems more something to raise
>> with the gcc folks in order to have a more "explicit" description that leaves no
>> room to the interpretation.
>>
>> ...
>>
>>>
>>> However, the workaround is cheap, and to avoid the chance of subtle
>>> intermittent code gen bugs it may be worth it:
>>>
>>> void foo(int x, int y)
>>> {
>>> 	asm volatile (
>>> 		"mov	x0, %0\n\t"
>>> 		"mov	x1, %1\n\t"
>>> 		"svc	#0"
>>> 		:: "r" (x), "r" (bar(x, y))
>>> 		: "r0", "r1", "memory"
>>> 	);
>>> }
>>>
>>> ->
>>>
>>> 0000000000000000 <foo>:
>>>    0:   a9be7bfd        stp     x29, x30, [sp, #-32]!
>>>    4:   910003fd        mov     x29, sp
>>>    8:   f9000bf3        str     x19, [sp, #16]
>>>    c:   2a0003f3        mov     w19, w0
>>>   10:   94000000        bl      0 <bar>
>>>   14:   2a0003e2        mov     w2, w0
>>>   18:   aa1303e0        mov     x0, x19
>>>   1c:   aa0203e1        mov     x1, x2
>>>   20:   d4000001        svc     #0x0
>>>   24:   f9400bf3        ldr     x19, [sp, #16]
>>>   28:   a8c27bfd        ldp     x29, x30, [sp], #32
>>>   2c:   d65f03c0        ret
>>>
>>>
>>> What do you think?
>>>
>>
>> The solution seems ok, thanks for providing it, but IMHO I think we
>> should find a workaround for something that is broken, which, unless
>> I am missing something major, this seems not the case.
> 
> So, after a bit of further experimentation, I found that I could trigger
> it with implicit function calls on an older compiler.  I couldn't show
> it with explicit function calls (as in your example).
> 
> With the following code, inlining if an expression that causes an
> implicit call to a libgcc helper can trigger this issue, but I had to
> try an older compiler:
> 
> int foo(int x, int y)
> {
> 	register int res asm("r0");
> 	register const int x_ asm("r0") = x;
> 	register const int y_ asm("r1") = y;
> 
> 	asm volatile (
> 		"svc	#0"
> 		: "=r" (res)
> 		: "r" (x_), "r" (y_)
> 		: "memory"
> 	);
> 
> 	return res;
> }
> 
> int bar(int x, int y)
> {
> 	return foo(x, x / y);
> }
> 
> -> (arm-linux-gnueabihf-gcc 9.1 -O2)
> 
> 00000000 <foo>:
>    0:   df00            svc     0
>    2:   4770            bx      lr
> 
> 00000004 <bar>:
>    4:   b510            push    {r4, lr}
>    6:   4604            mov     r4, r0
>    8:   f7ff fffe       bl      0 <__aeabi_idiv>
>    c:   4601            mov     r1, r0
>    e:   4620            mov     r0, r4
>   10:   df00            svc     0
>   12:   bd10            pop     {r4, pc}
> 
> -> (arm-linux-gnueabihf-gcc 5.1 -O2)
> 
> 00000000 <foo>:
>    0:   df00            svc     0
>    2:   4770            bx      lr
> 
> 00000004 <bar>:
>    4:   b508            push    {r3, lr}
>    6:   f7ff fffe       bl      0 <__aeabi_idiv>
>    a:   4601            mov     r1, r0
>    c:   df00            svc     0
>    e:   bd08            pop     {r3, pc}
> 

Thanks for reporting this. I had a go with gcc-5.1 on the vDSO library and seems
Ok, but it was worth trying.

For obvious reasons I am not reporting the objdump here :)

> I was struggling to find a way to emit an implicit function call for
> AArch64, except for 128-bit divide, which would complicate things since
> uint128_t doesn't fit in a single register anyway.
> 
> Maybe this was considered a bug and fixed sometime after GCC 5, but I
> think the GCC documentation is still quite unclear on the semantics of
> register asm vars that alias call-clobbered registers in the PCS.
> 
> If we can get a promise out of the GCC folks that this will not happen
> with any future compiler, then maybe we could just require a new enough
> compiler to be used.
> 

On this I fully agree, the compiler should never change an "expected" behavior.

If the issue comes from a gray area in the documentation, we have to address it
and have it fixed there.

The minimum version of the compiler from linux-4.19 is 4.6, hence I had to try
that the vDSO lib does not break with 5.1 [1].

[1]
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cafa0010cd51fb711fdcb50fc55f394c5f167a0a

> Then of course there is clang.
> 

I could not help myself and I tried clang.8 and clang.7 as well with my example,
just to make sure that we are fine even in that case. Please find below the
results (pretty identical).

main.clang.7.o:	file format ELF64-aarch64-little

Disassembly of section .text:
0000000000000000 show_it:
       0:	e8 03 1f aa 	mov	x8, xzr
       4:	09 68 68 38 	ldrb	w9, [x0, x8]
       8:	08 05 00 91 	add	x8, x8, #1
       c:	c9 ff ff 34 	cbz	w9, #-8 <show_it+0x4>
      10:	02 05 00 51 	sub	w2, w8, #1
      14:	e1 03 00 aa 	mov	x1, x0
      18:	08 08 80 d2 	mov	x8, #64
      1c:	01 00 00 d4 	svc	#0
      20:	c0 03 5f d6 	ret

main.clang.8.o:	file format ELF64-aarch64-little

Disassembly of section .text:
0000000000000000 show_it:
       0:	e8 03 1f aa 	mov	x8, xzr
       4:	09 68 68 38 	ldrb	w9, [x0, x8]
       8:	08 05 00 91 	add	x8, x8, #1
       c:	c9 ff ff 34 	cbz	w9, #-8 <show_it+0x4>
      10:	02 05 00 51 	sub	w2, w8, #1
      14:	e1 03 00 aa 	mov	x1, x0
      18:	08 08 80 d2 	mov	x8, #64
      1c:	01 00 00 d4 	svc	#0
      20:	c0 03 5f d6 	ret

Commands used:

$ clang -target aarch64-linux-gnueabi main.c -O -c -o main.clang.<x>.o
$ llvm-objdump -d main.clang.<x>.o

> Cheers
> ---Dave
> 

-- 
Regards,
Vincenzo