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

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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>
This commit is contained in:
Martin Schwidefsky
2016-10-27 12:41:39 +02:00
parent 45be0a02f8
commit 6e2ef5e4f6
12 changed files with 130 additions and 62 deletions
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15
arch/s390/kernel/early.c
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@@ -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)