The callbacks are based on Linux's implementation:
- CNTVCT_EL0 provides direct access to the system virtual timer[1].
- 'yield' serves as a CPU hint with similar semantics as x86's
'pause'[2].
In contrast with the kernel's implementation, there isn't a need for
isb() after reading CNTVCT_EL0, this is only needed in-kernel as the
register read has to be ordered with a subsequent locking operation.
[1] See Linux's '__arch_get_hw_counter()' in arch/arm64/include/asm/vdso/gettimeofday.h
[2] See Linux's 1baa82f4803 ("arm64: Implement cpu_relax as yield").
Signed-off-by: Nicolas Saenz Julienne <nsaenzju@xxxxxxxxxx>
--
Changes since v1:
- Code cleanup
- Add compiler barriers
src/oslat/oslat.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/src/oslat/oslat.c b/src/oslat/oslat.c
index 33cccd3..c90ec1a 100644
--- a/src/oslat/oslat.c
+++ b/src/oslat/oslat.c
@@ -71,6 +71,19 @@ static inline void frc(uint64_t *pval)
{
__asm__ __volatile__("mfspr %0, 268\n" : "=r" (*pval));
}
+# elif defined(__aarch64__)
+# define relax() __asm__ __volatile("yield" : : : "memory")
+
+static inline void frc(uint64_t *pval)
+{
+ /*
+ * This isb() is required to prevent that the counter value
+ * is speculated.
+ */
+ __asm__ __volatile__("isb" : : : "memory");
+ __asm__ __volatile__("mrs %0, cntvct_el0" : "=r" (*pval) :: "memory");
+
+}
# else
# define relax() do { } while (0)
# define frc(x)
--
2.31.1
