Re: linux-next: add utrace tree

From: Linus Torvalds
Date: Sat Jan 30 2010 - 13:00:51 EST

On Sat, 30 Jan 2010, Steven Rostedt wrote:
> The kernel is limited to what instructions it can perform, no floating
> point for example (of course there are some exceptions).

Actually, the reason the kernel is limited to not performing floating
point instructions is that teh kernel doesn't own the floating point
register set - it's too big to save/restore, so the kernel leaves it

But for emulating an instruction from user space, it would be perfectly
fine to do an FP instruction in kernel space, since we're explicitly doing
it on behalf of user space, and with user space owning it.

Of course, that would require that we _only_ touch the registers that user
space wants us to touch, which is likely impossible in practice for
anything but an execute-out-of-line model.

> But generally, the instructions in the kernel should be easier to
> emulate than in userspace.

Yeah, we control the kernel instructions better, and we know what the
environment is. For example, we never have to worry about vm86 mode or
segments when we fix up kernel instructions, but user space can do
anything, of course.

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