[PATCH v2] mips: add vdso support

[Hauke Mehrtens <hauke@hauke-m.de>]

vdso support is available on mips starting with kernel 4.4, see kernel
commit a7f4df4e21 "MIPS: VDSO: Add implementations of gettimeofday()
and clock_gettime()" for details.

In Linux kernel 4.4.0 the mips code returns -ENOSYS in case it can not
handle the vdso call and assumes the libc will call the original
syscall in this case. Handle this case in musl. Currently Linux kernel
4.4.0 handles the following types: CLOCK_REALTIME_COARSE,
CLOCK_MONOTONIC_COARSE, CLOCK_REALTIME and CLOCK_MONOTONIC.

These are some measurements of calling clock_gettime(CLOCK_MONOTONIC,
&tp); 1.000.000 times.

without vdso:
root@OpenWrt:/# time ./vdso-test
real 0m 0.95s
user 0m 0.24s
sys 0m 0.70s

with vdso:
root@OpenWrt:/# time /usr/bin/vdso-test
real 0m 0.35s
user 0m 0.34s
sys 0m 0.00s

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
---

I just saw that the mips stuff is implemented in a bad way, it returns
-ENOSYS when it can not handle the vdso call. I already complained to
the mips kernel developers. ;-) This code was not tested, I am still
building.

 arch/mips/syscall_arch.h |  4 ++++
 src/time/clock_gettime.c | 12 +++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/arch/mips/syscall_arch.h b/arch/mips/syscall_arch.h
index e74e0ad..39c0ea3 100644
--- a/arch/mips/syscall_arch.h
+++ b/arch/mips/syscall_arch.h
@@ -161,3 +161,7 @@ static inline long __syscall6(long n, long a, long b, long c, long d, long e, lo
 	if (n == SYS_fstatat) __stat_fix(c);
 	return r2;
 }
+
+#define VDSO_USEFUL
+#define VDSO_CGT_SYM "__vdso_clock_gettime"
+#define VDSO_CGT_VER "LINUX_2.6"
diff --git a/src/time/clock_gettime.c b/src/time/clock_gettime.c
index 1572de0..dba99ff 100644
--- a/src/time/clock_gettime.c
+++ b/src/time/clock_gettime.c
@@ -26,13 +26,23 @@ void *__vdsosym(const char *, const char *);
 int __clock_gettime(clockid_t clk, struct timespec *ts)
 {
 #ifdef VDSO_CGT_SYM
+	int ret;
 	static int (*volatile cgt)(clockid_t, struct timespec *);
 	if (!cgt) {
 		void *f = __vdsosym(VDSO_CGT_VER, VDSO_CGT_SYM);
 		if (!f) f = (void *)sc_clock_gettime;
 		a_cas_p(&cgt, 0, f);
 	}
-	return cgt(clk, ts);
+	ret = cgt(clk, ts);
+
+	/*
+	 * mips in linux kernel 4.4.0 returns -ENOSYS if it can not
+	 * handle the syscall in vdso, the original syscall should be
+	 * called by the libc in such a case.
+	 */
+	if (ret == -ENOSYS)
+		return sc_clock_gettime(clk, ts);
+	return ret;
 #else
 	return sc_clock_gettime(clk, ts);
 #endif
-- 
2.7.0.rc3