1. Name the kernel option XTRACE instead of EXCEPTION_TRACING
2. Put support functions in ia64/ia64/xtrace.c
3. Make it work with SMP by giving each CPU its own buffer
4. Save 16 key registers in the buffer for every exception
5. In ia64_handle_intr() and trap() transfer the trace record
to the KTR trace buffer using CTRx() and with some basic
information for now
6. Use a tunable to anble tracing and stop tracing as soon as
we enter the debugger
Room for improvements:
1. Transferring exception-relevant information to KTR
2. Add a sysctl to enable/disable tracing
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