Re: perf tools build broken for RISCV 32 bit

[Arnd Bergmann]

On Thu, Jan 14, 2021 at 7:38 PM Emiliano Ingrassia
<ingrassia@xxxxxxxxxxxxxx> wrote:
>
> Hi,
>
> When building perf for RISCV 32 bit (v5.10.7) I got the following
>
> | In file included from bench/futex-hash.c:29:
> | bench/futex.h: In function ‘futex_wait’:
> | bench/futex.h:37:10: error: ‘SYS_futex’ undeclared (first use in this function); did you mean ‘SYS_tee’?
>
> This issue is similar to the one reported in https://lkml.org/lkml/2019/4/19/631
>
> I found that patching tools/arch/riscv/include/uapi/asm/unistd.h as following:
>
>  #ifdef __LP64__
>  #define __ARCH_WANT_NEW_STAT
>  #define __ARCH_WANT_SET_GET_RLIMIT
> +#else
> +#define __ARCH_WANT_TIME32_SYSCALLS
>  #endif /* __LP64__ */
>
> solved the problem.
>
> I also found that a similar patch for arch/riscv/include/uapi/asm/unistd.h
> was removed in commit d4c08b9776b3, so probably this is not the right way(?).

In short, it won't work, as rv32 does not provide the time32 syscalls.
Your patch will make the application build, but it will not be able to
call futex().

You will in fact run into a related problem on any 32-bit architecture
if CONFIG_COMPAT_32BIT_TIME is disabled, or if you pass a non-NULL
timeout parameter and build with a time64-enabled libc.

The fix in the application is to call either __NR_futex or __NR_futex64
depending on the definition of time_t in the C library. I would recommend
doing it like

#ifdef __NR_futex
#define do_futex (sizeof(time_t) == sizeof(__kernel_long_t)) ? \
         __NR_futex : __NR_futex_time64
#else
#define do_futex __NR_futex
#done

       Arnd