[Xenomai] Mayday mechanism broken on x86-64 - simpler approach feasible?

[Jan Kiszka <jan.kiszka@siemens.com>]

Hi,

while porting the sigdebug test case to forge I noticed occasional
crashes of the test after it received a watchdog signal. It turned out
that the problem is in the way mayday works on x86-64:

1. sp/ip/ax of interrupted thread is saved
2. ip is set to mayday page
3. mayday page executes the mayday syscall via the syscall instruction
4. the syscall restores state on original state on return

That that's the theory. Unfortunately the syscall instructions
overwrites the cx register with the caller's instruction pointer. We
could save it but there is no way to restore it on syscall return
without significant changes to the Linux code in entry_64.S.

x86-32 is fine as it uses the int80 path which does a full state
recovery - but that is not available for 64-bit (only via ia32 compat,
but that can be turned off).

I was playing with a different approach: Set a flag that mayday is
underway, let the interrupt context cause a fault (set ip to 0) and then
fix that up if we are in mayday mode. Seems to work fine (after fixing
some x86 ipipe issue) and would simplify the mayday logic apparently. So
I'm wondering why that approach wasn't chosen initially? IOW, what am I
missing?

Jan

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