[PATCH 2/2] Introduce a vmonotonic_clock() vsyscall.

[Suleiman Souhlal <ssouhlal@FreeBSD.org>]

This is the vsyscall equivalent of monotonic_clock(), using the per-cpu vxtime.
It returns the number of nanoseconds since boot.

It's intended to be used by processes that need a cheap and nanosecond
resolution time counter.

Signed-off-by:	Suleiman Souhlal <suleiman@google.com>

---
 arch/x86_64/kernel/vsyscall.c |   21 +++++++++++++++++++++
 include/asm-x86_64/vsyscall.h |    1 +
 2 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/arch/x86_64/kernel/vsyscall.c b/arch/x86_64/kernel/vsyscall.c
index 9025699..f124cc6 100644
--- a/arch/x86_64/kernel/vsyscall.c
+++ b/arch/x86_64/kernel/vsyscall.c
@@ -52,6 +52,7 @@ #include <asm/unistd.h>
 
 #define	NS_SCALE 10
 #define NSEC_PER_TICK (NSEC_PER_SEC / HZ)
+extern unsigned long long monotonic_base;
 extern unsigned long hpet_tick;
 
 extern unsigned long vxtime_hz;
@@ -108,11 +109,13 @@ void vxtime_update_pcpu(void)
 
 	do {
 		seq = read_seqbegin(&xtime_lock);
+		vxtime.pcpu[cpu].monotonic_base = monotonic_base;
 		vxtime.pcpu[cpu].tv_sec = xtime.tv_sec;
 		vxtime.pcpu[cpu].tv_usec = xtime.tv_nsec / 1000;
 		offset = hpet_readl(HPET_COUNTER) - vxtime.last;
 	} while (read_seqretry(&xtime_lock, seq));
 
+	vxtime.pcpu[cpu].monotonic_base += offset * NSEC_PER_TICK / hpet_tick;
 	vxtime.pcpu[cpu].tv_usec += (offset * vxtime.quot) >> 32;
 	vxtime.pcpu[cpu].last_tsc = get_cycles_sync();
 
@@ -256,6 +259,24 @@ vgetcpu(unsigned *cpu, unsigned *node, s
 	return 0;
 }
 
+unsigned long __vsyscall(3) vmonotonic_clock(void)
+{
+	unsigned long nsec, seq, t;
+	int cpu;
+
+	do {
+		seq = read_seqbegin(&__vxtime.vx_seq);
+		cpu = apicid();
+		nsec = __vxtime.pcpu[cpu].monotonic_base;
+
+		rdtscll(t);
+		nsec += ((t - __vxtime.pcpu[cpu].last_tsc)
+		    * __vxtime.pcpu[cpu].tsc_nsquot) >> NS_SCALE;
+	} while (read_seqretry(&__vxtime.vx_seq, seq) || cpu != apicid());
+
+	return (nsec);
+}
+
 #ifdef CONFIG_SYSCTL
 
 #define SYSCALL 0x050f
diff --git a/include/asm-x86_64/vsyscall.h b/include/asm-x86_64/vsyscall.h
index 707353b..5a10fd0 100644
--- a/include/asm-x86_64/vsyscall.h
+++ b/include/asm-x86_64/vsyscall.h
@@ -31,6 +31,7 @@ #define VGETCPU_RDTSCP	1
 #define VGETCPU_LSL	2
 
 struct vxtime_pcpu {
+	unsigned long monotonic_base;
 	time_t tv_sec;
 	long tv_usec;
 	unsigned long tsc_nsquot;