[FFmpeg-devel] [PATCH] aarch64: Use regular hwcaps flags instead of HWCAP_CPUID for CPU feature detection on Linux

[Martin Storsjö]

On Wed, 28 Feb 2024, Martin Storsjö wrote:

> The CPU feature detection was added in
> 493fcde50a84cb23854335bcb0e55c6f383d55db, using HWCAP_CPUID.
>
> The argument for using that, was that HWCAP_CPUID was added much
> earlier in the kernel (in Linux v4.11), while the HWCAP flags for
> individual features were added much later. And if compiling with
> older userland headers that lack the bits for e.g. HWCAP_I8MM, we
> wouldn't be able to detect that feature.
>
> (In practice, e.g. Ubuntu 20.04 lacks HWCAP_I8MM in userland
> headers, but the toolchain does support assembling such
> instructions).
>
> However, while the flag HWCAP_I8MM was addded only in Linux v5.10,
> any CPU with that feature is most likely running a kernel that is
> newer than that as well. So by using HWCAP_CPUID, we could detect
> that feature on kernels between v4.11 and v5.10, but that is a
> quite unlikely case in practice.
>
> By using regular hwcaps flags, the code is much simplified, and
> doesn't rely on inline assembly to read the cpu id registers.
>
> And instead of requiring the userland headers to provide the
> definitions of the hwcap flags, provide our own definitions of the
> constants (they are fixed constants anyway), with names not conflicting
> with the ones from system headers. This avoids a number of ifdefs, and
> allows detecting these features even if building with userland headers
> that don't contain these definitions yet.
>
> Also, slightly older versions of QEMU, e.g. 6.2 in Ubuntu 22.04,
> do expose these features via HWCAP flags, but the emulated cpuid
> registers are missing the bits for exposing e.g. I8MM.
> ---
> libavutil/aarch64/cpu.c | 30 ++++++++----------------------
> 1 file changed, 8 insertions(+), 22 deletions(-)

Will apply on Monday, if there's no objections.

// Martin