1 files changed, 55 insertions, 0 deletions
diff --git a/library/std_detect/src/detect/os/openbsd/aarch64.rs b/library/std_detect/src/detect/os/openbsd/aarch64.rs
new file mode 100644
index 00000000000..cfe4ad10ad6
--- /dev/null
+++ b/library/std_detect/src/detect/os/openbsd/aarch64.rs
@@ -0,0 +1,55 @@
+//! Run-time feature detection for Aarch64 on OpenBSD.
+//!
+//! OpenBSD doesn't trap the mrs instruction, but exposes the system registers through sysctl.
+//! https://github.com/openbsd/src/commit/d335af936b9d7dd9cf655cae1ce19560c45de6c8
+//! https://github.com/golang/go/commit/cd54ef1f61945459486e9eea2f016d99ef1da925
+
+use crate::detect::cache;
+use core::{mem::MaybeUninit, ptr};
+
+// Defined in machine/cpu.h.
+// https://github.com/openbsd/src/blob/72ccc03bd11da614f31f7ff76e3f6fce99bc1c79/sys/arch/arm64/include/cpu.h#L25-L40
+const CPU_ID_AA64ISAR0: libc::c_int = 2;
+const CPU_ID_AA64ISAR1: libc::c_int = 3;
+const CPU_ID_AA64MMFR2: libc::c_int = 7;
+const CPU_ID_AA64PFR0: libc::c_int = 8;
+
+/// Try to read the features from the system registers.
+pub(crate) fn detect_features() -> cache::Initializer {
+    // ID_AA64ISAR0_EL1 and ID_AA64ISAR1_EL1 are supported on OpenBSD 7.1+.
+    // https://github.com/openbsd/src/commit/d335af936b9d7dd9cf655cae1ce19560c45de6c8
+    // Others are supported on OpenBSD 7.3+.
+    // https://github.com/openbsd/src/commit/c7654cd65262d532212f65123ee3905ba200365c
+    // sysctl returns an unsupported error if operation is not supported,
+    // so we can safely use this function on older versions of OpenBSD.
+    let aa64isar0 = sysctl64(&[libc::CTL_MACHDEP, CPU_ID_AA64ISAR0]).unwrap_or(0);
+    let aa64isar1 = sysctl64(&[libc::CTL_MACHDEP, CPU_ID_AA64ISAR1]).unwrap_or(0);
+    let aa64mmfr2 = sysctl64(&[libc::CTL_MACHDEP, CPU_ID_AA64MMFR2]).unwrap_or(0);
+    // Do not use unwrap_or(0) because in fp and asimd fields, 0 indicates that
+    // the feature is available.
+    let aa64pfr0 = sysctl64(&[libc::CTL_MACHDEP, CPU_ID_AA64PFR0]);
+
+    super::aarch64::parse_system_registers(aa64isar0, aa64isar1, aa64mmfr2, aa64pfr0)
+}
+
+#[inline]
+fn sysctl64(mib: &[libc::c_int]) -> Option<u64> {
+    const OUT_LEN: libc::size_t = core::mem::size_of::<u64>();
+    let mut out = MaybeUninit::<u64>::uninit();
+    let mut out_len = OUT_LEN;
+    let res = unsafe {
+        libc::sysctl(
+            mib.as_ptr(),
+            mib.len() as libc::c_uint,
+            out.as_mut_ptr() as *mut libc::c_void,
+            &mut out_len,
+            ptr::null_mut(),
+            0,
+        )
+    };
+    if res == -1 || out_len != OUT_LEN {
+        return None;
+    }
+    // SAFETY: we've checked that sysctl was successful and `out` was filled.
+    Some(unsafe { out.assume_init() })
+}