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

[Martin Storsjö]

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(-)

diff --git a/libavutil/aarch64/cpu.c b/libavutil/aarch64/cpu.c
index f27fef3992..7a05391343 100644
--- a/libavutil/aarch64/cpu.c
+++ b/libavutil/aarch64/cpu.c
@@ -24,34 +24,20 @@
 #include <stdint.h>
 #include <sys/auxv.h>
 
-#define get_cpu_feature_reg(reg, val) \
-        __asm__("mrs %0, " #reg : "=r" (val))
+#define HWCAP_AARCH64_ASIMDDP (1 << 20)
+#define HWCAP2_AARCH64_I8MM   (1 << 13)
 
 static int detect_flags(void)
 {
     int flags = 0;
 
-#if defined(HWCAP_CPUID) && HAVE_INLINE_ASM
     unsigned long hwcap = getauxval(AT_HWCAP);
-    // We can check for DOTPROD and I8MM using HWCAP_ASIMDDP and
-    // HWCAP2_I8MM too, avoiding to read the CPUID registers (which triggers
-    // a trap, handled by the kernel). However the HWCAP_* defines for these
-    // extensions are added much later than HWCAP_CPUID, so the userland
-    // headers might lack support for them even if the binary later is run
-    // on hardware that does support it (and where the kernel might support
-    // HWCAP_CPUID).
-    // See https://www.kernel.org/doc/html/latest/arm64/cpu-feature-registers.html
-    if (hwcap & HWCAP_CPUID) {
-        uint64_t tmp;
-
-        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
+    unsigned long hwcap2 = getauxval(AT_HWCAP2);
+
+    if (hwcap & HWCAP_AARCH64_ASIMDDP)
+        flags |= AV_CPU_FLAG_DOTPROD;
+    if (hwcap2 & HWCAP2_AARCH64_I8MM)
+        flags |= AV_CPU_FLAG_I8MM;
 
     return flags;
 }
-- 
2.39.3 (Apple Git-145)