x86-64: Move vread_tsc and vread_hpet into the vDSO

The vsyscall page now consists entirely of trap instructions.

Cc: John Stultz <johnstul@us.ibm.com>
Signed-off-by: Andy Lutomirski <luto@mit.edu>
Link: http://lkml.kernel.org/r/637648f303f2ef93af93bae25186e9a1bea093f5.1310639973.git.luto@mit.edu
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>

1 files changed, 47 insertions, 6 deletions

diff --git a/arch/x86/vdso/vclock_gettime.c b/arch/x86/vdso/vclock_gettime.c
index cf54813..8792d6e 100644
--- a/arch/x86/vdso/vclock_gettime.c
+++ b/arch/x86/vdso/vclock_gettime.c
@@ -17,6 +17,7 @@
 #include <linux/time.h>
 #include <linux/string.h>
 #include <asm/vsyscall.h>
+#include <asm/fixmap.h>
 #include <asm/vgtod.h>
 #include <asm/timex.h>
 #include <asm/hpet.h>
@@ -25,6 +26,43 @@
 
 #define gtod (&VVAR(vsyscall_gtod_data))
 
+notrace static cycle_t vread_tsc(void)
+{
+	cycle_t ret;
+	u64 last;
+
+	/*
+	 * Empirically, a fence (of type that depends on the CPU)
+	 * before rdtsc is enough to ensure that rdtsc is ordered
+	 * with respect to loads.  The various CPU manuals are unclear
+	 * as to whether rdtsc can be reordered with later loads,
+	 * but no one has ever seen it happen.
+	 */
+	rdtsc_barrier();
+	ret = (cycle_t)vget_cycles();
+
+	last = VVAR(vsyscall_gtod_data).clock.cycle_last;
+
+	if (likely(ret >= last))
+		return ret;
+
+	/*
+	 * GCC likes to generate cmov here, but this branch is extremely
+	 * predictable (it's just a funciton of time and the likely is
+	 * very likely) and there's a data dependence, so force GCC
+	 * to generate a branch instead.  I don't barrier() because
+	 * we don't actually need a barrier, and if this function
+	 * ever gets inlined it will generate worse code.
+	 */
+	asm volatile ("");
+	return last;
+}
+
+static notrace cycle_t vread_hpet(void)
+{
+	return readl((const void __iomem *)fix_to_virt(VSYSCALL_HPET) + 0xf0);
+}
+
 notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
 {
 	long ret;
@@ -36,9 +74,12 @@ notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
 notrace static inline long vgetns(void)
 {
 	long v;
-	cycles_t (*vread)(void);
-	vread = gtod->clock.vread;
-	v = (vread() - gtod->clock.cycle_last) & gtod->clock.mask;
+	cycles_t cycles;
+	if (gtod->clock.vclock_mode == VCLOCK_TSC)
+		cycles = vread_tsc();
+	else
+		cycles = vread_hpet();
+	v = (cycles - gtod->clock.cycle_last) & gtod->clock.mask;
 	return (v * gtod->clock.mult) >> gtod->clock.shift;
 }
 
@@ -118,11 +159,11 @@ notrace int __vdso_clock_gettime(clockid_t clock, struct timespec *ts)
 {
 	switch (clock) {
 	case CLOCK_REALTIME:
-		if (likely(gtod->clock.vread))
+		if (likely(gtod->clock.vclock_mode != VCLOCK_NONE))
 			return do_realtime(ts);
 		break;
 	case CLOCK_MONOTONIC:
-		if (likely(gtod->clock.vread))
+		if (likely(gtod->clock.vclock_mode != VCLOCK_NONE))
 			return do_monotonic(ts);
 		break;
 	case CLOCK_REALTIME_COARSE:
@@ -139,7 +180,7 @@ int clock_gettime(clockid_t, struct timespec *)
 notrace int __vdso_gettimeofday(struct timeval *tv, struct timezone *tz)
 {
 	long ret;
-	if (likely(gtod->clock.vread)) {
+	if (likely(gtod->clock.vclock_mode != VCLOCK_NONE)) {
 		if (likely(tv != NULL)) {
 			BUILD_BUG_ON(offsetof(struct timeval, tv_usec) !=
 				     offsetof(struct timespec, tv_nsec) ||