vmalloced stacks on x86_64?

[Andy Lutomirski]

Is there any good reason not to use vmalloc for x86_64 stacks?

The tricky bits I've thought of are:

 - On any context switch, we probably need to probe the new stack
before switching to it.  That way, if it's going to fault due to an
out-of-sync pgd, we still have a stack available to handle the fault.

 - Any time we change cr3, we may need to check that the pgd
corresponding to rsp is there.  If now, we need to sync it over.

 - For simplicity, we probably want all stack ptes to be present all
the time.  This is fine; vmalloc already works that way.

 - If we overrun the stack, we double-fault.  This should be easy to
detect: any double-fault where rsp is less than 20 bytes from the
bottom of the stack is a failure to deliver a non-IST exception due to
 a stack overflow.  The question is: what do we do if this happens?
We could just panic (guaranteed to work).  We could also try to
recover by killing the offending task, but that might be a bit
challenging, since we're in IST context.  We could do something truly
awful: increment RSP by a few hundred bytes, point RIP at do_exit, and
return from the double fault.

Thoughts?  This shouldn't be all that much code.

--Andy
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