Re: [FFmpeg-devel] [PATCH] lavu/riscv: do not fallback to AT_HWCAP auxillary vector

["Rémi Denis-Courmont" <remi@remlab.net>]

Le 20 juillet 2024 00:28:23 GMT+03:00, Brad Smith <brad-at-comstyle.com@ffmpeg.org> a écrit :
>On 2024-07-19 11:46 a.m., Rémi Denis-Courmont wrote:
>> If __riscv_hwprobe() fails, then the kernel version is presumably too
>> old. There is not much point falling back to the auxillary vector.
>> 
>> - The Linux kernel requires I, so the flag is always set on Linux, and
>>    run-time detection is unnecessary. Our RISC-V assembler does anyway not
>>    support targets without I.
>> 
>> - Linux can compile with or without F and D, but it cannot perform
>>    run-time detection for them (a kernel with F support will not boot a
>>    processor without F). The run-time detection is thus useless in that
>>    case. Besides F and D extensions are used throughout the C code, so
>>    their run-time detection would not be practical.
>> 
>> - Support for V was added in a later kernel version than riscv_hwprobe(),
>>    so the system call will always be available if the kernel supports V.
>>    The only exception would be vendor kernel forks, but those are known to
>>    haphasardly pretend to support V on systems without actual V support, or
>>    with only pre-ratification binary-incompatible version. Furthermore, a
>>    large chunk of our optimisations require Zba and/or Zbb which cannot be
>>    detected with HWCAP in those kernels.
>> 
>> For what it is worth, OpenJDK already took a similar action. Note that this
>> keeps AT_HWCAP usage for platforms with neither C run-time <sys/hwprobe.h>
>> nor kernel <asm/hwprobe.h>, notably kernels other than Linux.
>
>Thanks for the last part. The bits for OpenBSD have just landed for supporting
>elf_aux_info() so I'll look at adding support for FreeBSD and OpenBSD.

Well if they're using the `misa` CSR layout for HWCAP as Linux did, then there should be nothing to do, except maybe for compile fixes.

But if they use a custom bit-field layout then you probably need to rewrite the code.

>
>> ---
>>   libavutil/riscv/cpu.c | 5 ++---
>>   1 file changed, 2 insertions(+), 3 deletions(-)
>> 
>> diff --git a/libavutil/riscv/cpu.c b/libavutil/riscv/cpu.c
>> index 73abd289a6..04ac404bbf 100644
>> --- a/libavutil/riscv/cpu.c
>> +++ b/libavutil/riscv/cpu.c
>> @@ -83,9 +83,8 @@ int ff_get_cpu_flags_riscv(void)
>>                   break;
>>               default:
>>           }
>> -    } else
>> -#endif
>> -#if HAVE_GETAUXVAL
>> +    }
>> +#elif HAVE_GETAUXVAL
>>       {
>>           const unsigned long hwcap = getauxval(AT_HWCAP);
>>   
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