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kaiser: no slowdown if you run everything in kernelspace.

@djsundog i mean...

...

... yeah.

or, for that matter, if you manage to run on an entirely userspace IO stack, probably?

just as long as you don't cross those boundaries.

@djsundog ( maaajor caveat: if you're in a highly virtualized environment you've gotta make sure that you're staying in kernel space /all the way up the stack/.

trap-and-emulate anything will tank this pretty hard.

but. )

@gdkar @djsundog On IBM mainframes, you have an instruction called SIE, which stands for Start Interpreted Execution. This lets the processor nest virtual machines pretty much indefinitely with minimal impact to performance.

It works almost exactly like a call to Unix select(), where when SIE returns, you get not only the current state of the VM, but also what caused the instruction to "return."

SIE doesn't need to be in supervisor state to run.

We can learn a lot from mainframes.

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