s390/time: add support for the TOD clock epoch extension

The TOD epoch extension adds 8 epoch bits to the TOD clock to provide
a continuous clock after 2042/09/17. The store-clock-extended (STCKE)
instruction will store the epoch index in the first byte of the
16 bytes stored by the instruction. The read_boot_clock64 and the
read_presistent_clock64 functions need to take the additional bits
into account to give the correct result after 2042/09/17.

The clock-comparator register will stay 64 bit wide. The comparison
of the clock-comparator with the TOD clock is limited to bytes
1 to 8 of the extended TOD format. To deal with the overflow problem
due to an epoch change the clock-comparator sign control in CR0 can
be used to switch the comparison of the 64-bit TOD clock with the
clock-comparator to a signed comparison.

The decision between the signed vs. unsigned clock-comparator
comparisons is done at boot time. Only if the TOD clock is in the
second half of a 142 year epoch the signed comparison is used.
This solves the epoch overflow issue as long as the machine is
booted at least once in an epoch.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 45 insertions, 20 deletions

diff --git a/arch/s390/kernel/time.c b/arch/s390/kernel/time.c
index 192efdfac918..15abecba068e 100644
--- a/arch/s390/kernel/time.c
+++ b/arch/s390/kernel/time.c
@@ -51,8 +51,15 @@
 #include <asm/cio.h>
 #include "entry.h"
 
-u64 sched_clock_base_cc = -1;	/* Force to data section. */
-EXPORT_SYMBOL_GPL(sched_clock_base_cc);
+unsigned char tod_clock_base[16] __aligned(8) = {
+	/* Force to data section. */
+	0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+	0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff
+};
+EXPORT_SYMBOL_GPL(tod_clock_base);
+
+u64 clock_comparator_max = -1ULL;
+EXPORT_SYMBOL_GPL(clock_comparator_max);
 
 static DEFINE_PER_CPU(struct clock_event_device, comparators);
 
@@ -75,7 +82,7 @@ void __init time_early_init(void)
 	struct ptff_qui qui;
 
 	/* Initialize TOD steering parameters */
-	tod_steering_end = sched_clock_base_cc;
+	tod_steering_end = *(unsigned long long *) &tod_clock_base[1];
 	vdso_data->ts_end = tod_steering_end;
 
 	if (!test_facility(28))
@@ -111,22 +118,27 @@ unsigned long long monotonic_clock(void)
 }
 EXPORT_SYMBOL(monotonic_clock);
 
-static void tod_to_timeval(__u64 todval, struct timespec64 *xt)
+static void ext_to_timespec64(unsigned char *clk, struct timespec64 *xt)
 {
-	unsigned long long sec;
+	unsigned long long high, low, rem, sec, nsec;
+
+	/* Split extendnd TOD clock to micro-seconds and sub-micro-seconds */
+	high = (*(unsigned long long *) clk) >> 4;
+	low = (*(unsigned long long *)&clk[7]) << 4;
+	/* Calculate seconds and nano-seconds */
+	sec = high;
+	rem = do_div(sec, 1000000);
+	nsec = (((low >> 32) + (rem << 32)) * 1000) >> 32;
 
-	sec = todval >> 12;
-	do_div(sec, 1000000);
 	xt->tv_sec = sec;
-	todval -= (sec * 1000000) << 12;
-	xt->tv_nsec = ((todval * 1000) >> 12);
+	xt->tv_nsec = nsec;
 }
 
 void clock_comparator_work(void)
 {
 	struct clock_event_device *cd;
 
-	S390_lowcore.clock_comparator = -1ULL;
+	S390_lowcore.clock_comparator = clock_comparator_max;
 	cd = this_cpu_ptr(&comparators);
 	cd->event_handler(cd);
 }
@@ -148,7 +160,7 @@ void init_cpu_timer(void)
 	struct clock_event_device *cd;
 	int cpu;
 
-	S390_lowcore.clock_comparator = -1ULL;
+	S390_lowcore.clock_comparator = clock_comparator_max;
 	set_clock_comparator(S390_lowcore.clock_comparator);
 
 	cpu = smp_processor_id();
@@ -179,7 +191,7 @@ static void clock_comparator_interrupt(struct ext_code ext_code,
 				       unsigned long param64)
 {
 	inc_irq_stat(IRQEXT_CLK);
-	if (S390_lowcore.clock_comparator == -1ULL)
+	if (S390_lowcore.clock_comparator == clock_comparator_max)
 		set_clock_comparator(S390_lowcore.clock_comparator);
 }
 
@@ -197,18 +209,28 @@ static void stp_reset(void);
 
 void read_persistent_clock64(struct timespec64 *ts)
 {
-	__u64 clock;
+	unsigned char clk[STORE_CLOCK_EXT_SIZE];
+	__u64 delta;
 
-	clock = get_tod_clock() - initial_leap_seconds;
-	tod_to_timeval(clock - TOD_UNIX_EPOCH, ts);
+	delta = initial_leap_seconds + TOD_UNIX_EPOCH;
+	get_tod_clock_ext(clk);
+	*(__u64 *) &clk[1] -= delta;
+	if (*(__u64 *) &clk[1] > delta)
+		clk[0]--;
+	ext_to_timespec64(clk, ts);
 }
 
 void read_boot_clock64(struct timespec64 *ts)
 {
-	__u64 clock;
+	unsigned char clk[STORE_CLOCK_EXT_SIZE];
+	__u64 delta;
 
-	clock = sched_clock_base_cc - initial_leap_seconds;
-	tod_to_timeval(clock - TOD_UNIX_EPOCH, ts);
+	delta = initial_leap_seconds + TOD_UNIX_EPOCH;
+	memcpy(clk, tod_clock_base, 16);
+	*(__u64 *) &clk[1] -= delta;
+	if (*(__u64 *) &clk[1] > delta)
+		clk[0]--;
+	ext_to_timespec64(clk, ts);
 }
 
 static u64 read_tod_clock(struct clocksource *cs)
@@ -406,7 +428,10 @@ static void clock_sync_global(unsigned long long delta)
 	struct ptff_qto qto;
 
 	/* Fixup the monotonic sched clock. */
-	sched_clock_base_cc += delta;
+	*(unsigned long long *) &tod_clock_base[1] += delta;
+	if (*(unsigned long long *) &tod_clock_base[1] < delta)
+		/* Epoch overflow */
+		tod_clock_base[0]++;
 	/* Adjust TOD steering parameters. */
 	vdso_data->tb_update_count++;
 	now = get_tod_clock();
@@ -437,7 +462,7 @@ static void clock_sync_global(unsigned long long delta)
 static void clock_sync_local(unsigned long long delta)
 {
 	/* Add the delta to the clock comparator. */
-	if (S390_lowcore.clock_comparator != -1ULL) {
+	if (S390_lowcore.clock_comparator != clock_comparator_max) {
 		S390_lowcore.clock_comparator += delta;
 		set_clock_comparator(S390_lowcore.clock_comparator);
 	}