s390/wti: Add wti accounting for missed grace periods

s390/wti: Add wti accounting for missed grace periods

A virtual CPU that has received a warning-track interrupt may fail to
acknowledge the interrupt within the warning-track grace period.
While this is usually not a problem, it will become necessary to
investigate if there is a large number of such missed warning-track
interrupts. Therefore, it is necessary to track these events.
The information is tracked through the s390 debug facility and can be
found under /sys/kernel/debug/s390dbf/wti/.

The hex_ascii output is formatted as:
 <pid> <symbol>

The values pid and current psw are collected when a warning track
interrupt is received. Symbol is either the kernel symbol matching the
collected psw or redacted to <user> when running in user space.

Each line represents the currently executing process when a warning
track interrupt was received which was then not acknowledged within its
grace period.

Acked-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Mete Durlu <meted@linux.ibm.com>
Signed-off-by: Tobias Huschle <huschle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>