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, 11 insertions, 4 deletions

diff --git a/arch/s390/kernel/early.c b/arch/s390/kernel/early.c
index 5d20182ee8ae..added6790460 100644
--- a/arch/s390/kernel/early.c
+++ b/arch/s390/kernel/early.c
@@ -53,8 +53,9 @@ static void __init reset_tod_clock(void)
 	if (set_tod_clock(TOD_UNIX_EPOCH) != 0 || store_tod_clock(&time) != 0)
 		disabled_wait(0);
 
-	sched_clock_base_cc = TOD_UNIX_EPOCH;
-	S390_lowcore.last_update_clock = sched_clock_base_cc;
+	memset(tod_clock_base, 0, 16);
+	*(__u64 *) &tod_clock_base[1] = TOD_UNIX_EPOCH;
+	S390_lowcore.last_update_clock = TOD_UNIX_EPOCH;
 }
 
 #ifdef CONFIG_SHARED_KERNEL
@@ -165,8 +166,8 @@ static noinline __init void create_kernel_nss(void)
 	}
 
 	/* re-initialize cputime accounting. */
-	sched_clock_base_cc = get_tod_clock();
-	S390_lowcore.last_update_clock = sched_clock_base_cc;
+	get_tod_clock_ext(tod_clock_base);
+	S390_lowcore.last_update_clock = *(__u64 *) &tod_clock_base[1];
 	S390_lowcore.last_update_timer = 0x7fffffffffffffffULL;
 	S390_lowcore.user_timer = 0;
 	S390_lowcore.system_timer = 0;
@@ -387,6 +388,12 @@ static __init void detect_machine_facilities(void)
 	}
 	if (test_facility(133))
 		S390_lowcore.machine_flags |= MACHINE_FLAG_GS;
+	if (test_facility(139) && (tod_clock_base[1] & 0x80)) {
+		/* Enabled signed clock comparator comparisons */
+		S390_lowcore.machine_flags |= MACHINE_FLAG_SCC;
+		clock_comparator_max = -1ULL >> 1;
+		__ctl_set_bit(0, 53);
+	}
 }
 
 static inline void save_vector_registers(void)