These changes are paired with updates to lockdep+ftrace in order to
keep the (perceived) IRQ state consistent with the expectations of the
latter.

Bottom line: IRQ events which preempt the root stage while stalled
must not alter the tracer state incorrectly.

The change in #5a7e6eeede58 does not fix the root issue, but causes
__ipipe_tsc_register() to refer to .init data which is a section
violation too. Unfortunately, __ipipe_tsc_register() may be called
long after boot when loading a clocksource dynamically (which is what
the above commit was trying to address), but the position of TSC
helpers in the KUSER page depends on other -unrelated- helpers so we
have to define them in the same section.

To escape this stalemate, move the whole KUSER setup code and
variables outside of the .init section (TSC and other helpers), which
should only amount to a few hundreds of bytes.

CONFIG_DEBUG_SECTION_MISMATCH=y reports:

The function dw_apb_clocksource_register() references
the function __init __ipipe_tsc_register().
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

This adds the arm-specific bits to implement the userspace return
notifier.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

Although the regular SMP code is fine with keeping IPI_CPU_BACKTRACE
out of this count, we don't as we depend on it to figure out the exact
number of IPI vectors in use. The last IPI number we must allocate is
NR_IPI + IPIPE_LAST_IPI + 1, to account for the empty CPU_BACKTRACE
slot.

This bug badly breaks ipipe_raise_irq() by causing a conflict in the
VIRQ namespace between the last IPI and the first allocated VIRQ
allocated by the generic code (detected with deferred out-of-band
printk() messages not showing up).

Something moved the value of this constant but didn't update the
hard-coded instances. Fix this and avoid future problems by using the
proper symbolic value.

That just requires harding asm/ipipe_base.h for assembly use.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

The mayday trap asks the co-kernel to deal with a runaway context
which has been detected earlier. When doing so, the co-kernel might
re-enable hard interrupts, which we have to disable again before
unwinding the IRQ context which triggered the whole process (like a
watchdog event).

Use __ipipe_call_mayday() to fire the notification to the co-kernel
instead of the open coded version, so that we do restore the hard
interrupt state properly before leaving __ipipe_exit_irq().