[PATCH v2 2/3] oslat: Add aarch64 support

[Nicolas Saenz Julienne <nsaenzju@redhat.com>]

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@redhat.com>
--

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