1 files changed, 45 insertions, 0 deletions
diff --git a/src/time.rs b/src/time.rs
new file mode 100644
index 0000000..eff567d
--- /dev/null
+++ b/src/time.rs
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+//! Functions to read time stamp counters on x86.
+
+/// Read the time stamp counter.
+///
+/// The RDTSC instruction is not a serializing instruction.
+/// It does not necessarily wait until all previous instructions
+/// have been executed before reading the counter. Similarly,
+/// subsequent instructions may begin execution before the
+/// read operation is performed. If software requires RDTSC to be
+/// executed only after all previous instructions have completed locally,
+/// it can either use RDTSCP or execute the sequence LFENCE;RDTSC.
+///
+/// # Safety
+/// * Causes a GP fault if the TSD flag in register CR4 is set and the CPL
+///   is greater than 0.
+#[allow(unused_mut)]
+pub unsafe fn rdtsc() -> u64 {
+    let mut low: u32;
+    let mut high: u32;
+
+    asm!("rdtsc" : "={eax}" (low), "={edx}" (high));
+    ((high as u64) << 32) | (low as u64)
+}
+
+/// Read the time stamp counter.
+///
+/// The RDTSCP instruction waits until all previous instructions
+/// have been executed before reading the counter.
+/// However, subsequent instructions may begin execution
+/// before the read operation is performed.
+///
+/// Volatile is used here because the function may be used to act as
+/// an instruction barrier.
+///
+/// # Safety
+/// * Causes a GP fault if the TSD flag in register CR4 is set and the
+///   CPL is greater than 0.
+#[allow(unused_mut)]
+pub unsafe fn rdtscp() -> u64 {
+    let mut low: u32;
+    let mut high: u32;
+
+    asm!("rdtscp" : "={eax}" (low), "={edx}" (high) ::: "volatile");
+    ((high as u64) << 32) | (low as u64)
+}