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

Vincenzo Frascino
2019-06-26T13:28:09Z

Hi Dave,

On 25/06/2019 16:33, Dave Martin wrote:
> On Fri, Jun 21, 2019 at 10:52:31AM +0100, Vincenzo Frascino wrote:
>> To take advantage of the commonly defined vdso interface for
>> gettimeofday the architectural code requires an adaptation.
>>
>> Re-implement the gettimeofday vdso in C in order to use lib/vdso.
>>
>> With the new implementation arm64 gains support for CLOCK_BOOTTIME
>> and CLOCK_TAI.
>>
>> Cc: Catalin Marinas <catalin.marinas@arm.com>
>> Cc: Will Deacon <will.deacon@arm.com>
>> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.com>
>> Tested-by: Shijith Thotton <sthotton@marvell.com>
>> Tested-by: Andre Przywara <andre.przywara@arm.com>
>
> [...]
>
>> diff --git a/arch/arm64/include/asm/vdso/gettimeofday.h b/arch/arm64/include/asm/vdso/gettimeofday.h
>> new file mode 100644
>> index 000000000000..bc3cb6738051
>> --- /dev/null
>> +++ b/arch/arm64/include/asm/vdso/gettimeofday.h
>> @@ -0,0 +1,86 @@
>> +/* SPDX-License-Identifier: GPL-2.0 */
>> +/*
>> + * Copyright (C) 2018 ARM Limited
>> + */
>> +#ifndef __ASM_VDSO_GETTIMEOFDAY_H
>> +#define __ASM_VDSO_GETTIMEOFDAY_H
>> +
>> +#ifndef __ASSEMBLY__
>> +
>> +#include <asm/unistd.h>
>> +#include <uapi/linux/time.h>
>> +
>> +#define VDSO_HAS_CLOCK_GETRES 1
>> +
>> +static __always_inline int gettimeofday_fallback(
>> + struct __kernel_old_timeval *_tv,
>> + struct timezone *_tz)
>
> Out of interest, does this need to be __always_inline?
>

It is a design choice. Philosophically, I prefer to control and reduce the scope
of the decisions the compiler has to make in order to not have surprises.

>> +{
>> + register struct timezone *tz asm("x1") = _tz;
>> + register struct __kernel_old_timeval *tv asm("x0") = _tv;
>> + register long ret asm ("x0");
>> + register long nr asm("x8") = __NR_gettimeofday;
>> +
>> + asm volatile(
>> + " svc #0\n"
>
> Can inlining of this function result in non-trivial expressions being
> substituted for _tz or _tv?
>
> A function call can clobber register asm vars that are assigned to the
> caller-save registers or that the PCS uses for function arguments, and
> the situations where this can happen are poorly defined AFAICT. There's
> also no reliable way to detect at build time whether the compiler has
> done this, and no robust way to stop if happening.
>
> (IMHO the compiler is wrong to do this, but it's been that way for ever,
> and I think I saw GCC 9 show this behaviour recently when I was
> investigating something related.)
>
>
> To be safe, it's better to put this out of line, or remove the reg asm()
> specifiers, mark x0-x18 and lr as clobbered here (so that the compiler
> doesn't map arguments to them), and put movs in the asm to move things
> into the right registers. The syscall number can be passed with an "i"
> constraint. (And yes, this sucks.)
>
> If the code this is inlined in is simple enough though, we can be fairly
> confident of getting away with it.
>

I took very seriously what you are mentioning here because I think that
robustness of the code comes before than everything especially in the kernel and
I carried on some experiments to try to verify if in this case is safe to assume
that the compiler is doing the right thing.

Based on my investigation and on previous observations of the generation of the
vDSO library, I can conclude that the approach seems safe due to the fact that
the usage of this code is very limited, the code itself is simple enough and
that gcc would inline this code anyway based on the current compilation options.

The experiment that I did was to define some self-contained code that tries to
mimic what you are describing and compile it with 3 different versions of gcc
(6.4, 8.1 and 8.3) and in all the tree cases the behavior seems correct.

Code:
=====

typedef int ssize_t;
typedef int size_t;

static int my_strlen(const char *s)
{
int i = 0;

while (s[i] == '\0')
i++;

return i;
}

static inline ssize_t my_syscall(int fd, const void *buf, size_t count)
{
register ssize_t arg1 asm ("x0") = fd;
register const void *arg2 asm ("x1") = buf;
register size_t arg3 asm ("x2") = count;

__asm__ volatile (
"mov x8, #64\n"
"svc #0\n"
: "=&r" (arg1)
: "r" (arg2), "r" (arg3)
: "x8"
);

return arg1;
}

void sys_caller(const char *s)
{
my_syscall(1, s, my_strlen(s));
}

GCC 8.3.0:
==========

main.8.3.0.o: file format elf64-littleaarch64

Disassembly of section .text:

0000000000000000 <sys_caller>:
0: 39400001 ldrb w1, [x0]
4: 35000161 cbnz w1, 30 <sys_caller+0x30>
8: d2800023 mov x3, #0x1 // #1
c: d1000404 sub x4, x0, #0x1
10: 2a0303e2 mov w2, w3
14: 91000463 add x3, x3, #0x1
18: 38636881 ldrb w1, [x4, x3]
1c: 34ffffa1 cbz w1, 10 <sys_caller+0x10>
20: aa0003e1 mov x1, x0
24: d2800808 mov x8, #0x40 // #64
28: d4000001 svc #0x0
2c: d65f03c0 ret
30: 52800002 mov w2, #0x0 // #0
34: 17fffffb b 20 <sys_caller+0x20>

GCC 8.1.0:
==========

main.8.1.0.o: file format elf64-littleaarch64

Disassembly of section .text:

0000000000000000 <sys_caller>:
0: 39400001 ldrb w1, [x0]
4: 35000161 cbnz w1, 30 <sys_caller+0x30>
8: d2800023 mov x3, #0x1 // #1
c: d1000404 sub x4, x0, #0x1
10: 2a0303e2 mov w2, w3
14: 91000463 add x3, x3, #0x1
18: 38636881 ldrb w1, [x4, x3]
1c: 34ffffa1 cbz w1, 10 <sys_caller+0x10>
20: aa0003e1 mov x1, x0
24: d2800808 mov x8, #0x40 // #64
28: d4000001 svc #0x0
2c: d65f03c0 ret
30: 52800002 mov w2, #0x0 // #0
34: 17fffffb b 20 <sys_caller+0x20>

GCC 6.4.0:
==========

main.6.4.0.o: file format elf64-littleaarch64

Disassembly of section .text:

0000000000000000 <sys_caller>:
0: 39400001 ldrb w1, [x0]
4: 35000161 cbnz w1, 30 <sys_caller+0x30>
8: d2800023 mov x3, #0x1 // #1
c: d1000404 sub x4, x0, #0x1
10: 2a0303e2 mov w2, w3
14: 91000463 add x3, x3, #0x1
18: 38636881 ldrb w1, [x4, x3]
1c: 34ffffa1 cbz w1, 10 <sys_caller+0x10>
20: aa0003e1 mov x1, x0
24: d2800808 mov x8, #0x40 // #64
28: d4000001 svc #0x0
2c: d65f03c0 ret
30: 52800002 mov w2, #0x0 // #0
34: 17fffffb b 20 <sys_caller+0x20>

> [...]
>
> Cheers
> ---Dave
>

--
Regards,
Vincenzo