Re: Dealing with the NMI mess

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jul 23, 2015 at 01:38:33PM -0700, Linus Torvalds wrote:

> And the "take them and disable them" is really simple. No "am I in an
> NMI contect" thing (because that leads to the whole question about
> "what is NMI context"). That's not the real rule anyway.
> 
> No, make it very simple and straightforward. Make the test be "uhhuh,
> I got a #DB in kernel mode, and interrupts were disabled - I know I'm
> going to return with "ret", so I'm just going to have to disable this
> breakpoint".
> 
> Nothing clever. Nothing subtle. Nothing that needs "this range of
> instructions is magical". No.  Just a very simple rule: if the context
> we return to is kernel mode and interrupts are disabled, we're using
> 'ret', so we cannot suppress debug faults.
> 
> Did I miss something? There were a lot of emails flying around, but I
> *thought* I saw them all..

So the NMI could trigger userspace debug register faults, and simply
disabling them would make the whole debug register thing entirely
unreliable.