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

[Martin Storsjö]

ffmpeg | branch: master | Martin Storsjö <martin at martin.st> | Wed Feb 14 23:00:54 2024 +0200| [e30369bc1c683aeab6ea74bc37b4ae77b03f79b5] | committer: Martin Storsjö

aarch64: Use regular hwcaps flags instead of HWCAP_CPUID for CPU feature detection on Linux

This makes the code much simpler (especially for adding support
for other instruction set extensions), avoids needing inline
assembly for this feature, and generally is more of the canonical
way to do this.

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 always come later. This allows detecting support
for new CPU extensions before the kernel exposes information about
them via hwcap flags.

However in practice, there's probably quite little advantage in this.
E.g. HWCAP2_I8MM was added in Linux v5.10 - long after HWCAP_CPUID,
but there's probably very little practical cases where one would
run a kernel older than that on a CPU that supports those instructions.

Additionally, we provide our own definitions of the flag values to
check (as they are fixed constants anyway), with names not conflicting
with the ones from system headers. This reduces the number of ifdefs
needed, and allows detecting those features even if building with
userland headers that are lacking the definitions of those flags.

Also, slightly older versions of QEMU, e.g. 6.2 in Ubuntu 22.04,
do expose support for these features via HWCAP flags, but the
emulated cpuid registers are missing the bits for exposing e.g. I8MM.
(This issue is fixed in later versions of QEMU though.)

Signed-off-by: Martin Storsjö <martin at martin.st>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=e30369bc1c683aeab6ea74bc37b4ae77b03f79b5
---

 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;
 }