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Subjectvmalloced stacks on x86_64?
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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