On Fri, Feb 24, 2017 at 08:43:25AM +0100, Ingo Molnar wrote: > The only high level question is whether we trust the trap machinery to > generate > WARN_ON()s. I believe we do. > > BTW.: why not use INT3 instead of all these weird #UD opcodes? It's a single > byte > opcode and we can do a quick exception table search in do_debug(). This way > we'll > also have irqs disabled which might help getting the message out before any > irq > handler comes in and muddies the waters. > > In a sense WARN_ON()s and BUG_ON()s can be considered permanently installed > in-line kprobes, with a special, built-in handler.
I've actually been looking into that. There's a bunch of 'fun' details that I've been checking, but I think I can make that happen. My initial patch extended the existing UD2 BUG trap to include the WARN, this is what many other architectures already do. Arjan then complained that some emulators terminate on UD2 and could I please not use that for WARN, at which point Borislav called my attention to UD0/UD1. So I made the UD0 change and posted (fwiw, there's a lost refresh in the patch I posted and it will not actually work). I think I'll post an update of said patch and then attempt to do the INT3 thing in a later patch -- that will require at least one new knob in the generic BUG code ... > BTW. #2: side note, GCC generated crap code here. Why didn't it do: I've seen GCC do 'wonderful' things the past few weeks. Absolutely mind boggling stuff.
