Re: [FFmpeg-devel] [PATCH 3/4] aarch64: Add linux runtime cpu feature detection using getauxval(AT_HWCAP)

From: "Martin Storsjö" <martin@martin.st>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 3/4] aarch64: Add linux runtime cpu feature detection using getauxval(AT_HWCAP)
Date: Sat, 27 May 2023 23:35:14 +0300 (EEST)
Message-ID: <15bbcbd0-41ba-976e-61f0-3211c2529f@martin.st> (raw)
In-Reply-To: <2630251.OcK1j5fZQb@basile.remlab.net>

On Sat, 27 May 2023, Rémi Denis-Courmont wrote:

> Le perjantaina 26. toukokuuta 2023, 11.03.14 EEST Martin Storsjö a écrit :
>> Based on code by Janne Grunau.
>> 
>> Using HWCAP_CPUID for user space access to the CPU feature registers. See
>> https://www.kernel.org/doc/html/latest/arm64/cpu-feature-registers.html.
>> ---
>>  configure               |  2 ++
>>  libavutil/aarch64/cpu.c | 38 ++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 40 insertions(+)
>> 
>> diff --git a/configure b/configure
>> index 3c7473efb2..b5357b8d27 100755
>> --- a/configure
>> +++ b/configure
>> @@ -2207,6 +2207,7 @@ HAVE_LIST_PUB="
>>
>>  HEADERS_LIST="
>>      arpa_inet_h
>> +    asm_hwcap_h
>>      asm_types_h
>>      cdio_paranoia_h
>>      cdio_paranoia_paranoia_h
>> @@ -6422,6 +6423,7 @@ check_headers io.h
>>  enabled libdrm &&
>>      check_headers linux/dma-buf.h
>> 
>> +check_headers asm/hwcap.h
>>  check_headers linux/perf_event.h
>>  check_headers libcrystalhd/libcrystalhd_if.h
>>  check_headers malloc.h
>> diff --git a/libavutil/aarch64/cpu.c b/libavutil/aarch64/cpu.c
>> index 42b33e4a2d..34c838c2f5 100644
>> --- a/libavutil/aarch64/cpu.c
>> +++ b/libavutil/aarch64/cpu.c
>> @@ -20,6 +20,42 @@
>>  #include "libavutil/cpu_internal.h"
>>  #include "config.h"
>> 
>> +#if (defined(__linux__) || defined(__ANDROID__)) && HAVE_GETAUXVAL &&
>> HAVE_ASM_HWCAP_H +#include <stdint.h>
>> +#include <asm/hwcap.h>
>> +#include <sys/auxv.h>
>> +
>> +#define get_cpu_feature_reg(reg, val) \
>> +        __asm__("mrs %0, " #reg : "=r" (val))
>
> Strictly speaking, this can read any system register. One way to prevent that 
> would be to include the ID_ prefix and _EL1 suffix in the macro. I would have 
> used a pure static inline instead, but that's just a matter of taste.
>
>> +
>> +static int detect_flags(void)
>> +{
>> +    unsigned long ret = getauxval(AT_HWCAP);
>> +    int flags = 0;
>> +#if defined(HWCAP_CPUID)
>> +    uint64_t tmp;
>> +    if (!(ret & HWCAP_CPUID))
>> +        return flags;
>> +    get_cpu_feature_reg(ID_AA64ISAR0_EL1, tmp);
>> +    if (((tmp >> 44) & 0xf) == 0x1)
>> +        flags |= AV_CPU_FLAG_DOTPROD;
>> +    get_cpu_feature_reg(ID_AA64ISAR1_EL1, tmp);
>> +    if (((tmp >> 52) & 0xf) == 0x1)
>> +        flags |= AV_CPU_FLAG_I8MM;
>> +#endif
>
> NEON detection could be added here, though I've yet to see an Armv8 
> implementation without AdvSIMD.

I guess we could, but as it's part of the require baseline for armv8-a I 
don't think there's much need for it? If configured with --disable-neon we 
don't return that cpuflag though.

> FWIW, DotProd is exposed as HWCAP_ASIMDDP and I8MM is exposed via 
> HWCAP2_I8MM, using trapped ID registers is not (yet) necessary.

Ah, I see. I guess using those would be more straightforward.

OTOH, HWCAP_CPUID is available much earlier than HWCAP_ASIMDDP or 
HWCAP2_I8MM (I do some amount of cross building with a fairly old 
sysroot). I'll think about it, whether it's worth complicating things to 
try both approaches, or if we should just go with the plain HWCAPs here.

// Martin
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