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

From: "Rémi Denis-Courmont" <remi@remlab.net>
To: 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 12:04:15 +0300
Message-ID: <2630251.OcK1j5fZQb@basile.remlab.net> (raw)
In-Reply-To: <20230526080315.83424-3-martin@martin.st>

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.

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

> +
> +    return flags;
> +}
> +
> +#else
> +
> +static int detect_flags(void)
> +{
> +    return 0;
> +}
> +
> +#endif
> +
>  int ff_get_cpu_flags_aarch64(void)
>  {
>      int flags = AV_CPU_FLAG_ARMV8 * HAVE_ARMV8 |
> @@ -33,6 +69,8 @@ int ff_get_cpu_flags_aarch64(void)
>      flags |= AV_CPU_FLAG_I8MM;
>  #endif
> 
> +    flags |= detect_flags();
> +
>      return flags;
>  }

-- 
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http://www.remlab.net/

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