Re: [patch 1/2] x86_64 page fault NMI-safe

Mathieu Desnoyers
2010-07-14T20:17:57Z

* Linus Torvalds (torvalds@linux-foundation.org) wrote:
> On Wed, Jul 14, 2010 at 12:36 PM, Frederic Weisbecker
> <fweisbec@gmail.com> wrote:
> >
> > There is also the fact we need to handle the lost NMI, by defering its
> > treatment or so. That adds even more complexity.
>
> I don't think your read my proposal very deeply. It already handles
> them by taking a fault on the iret of the first one (that's why we
> point to the stack frame - so that we can corrupt it and force a
> fault).

It only handles the case of a single NMI coming in. What happens in this
scenario?

- NMI (1) comes in.
- takes a fault
- iret
- NMI (2) comes in.
- nesting detected, popf/ret
- takes another fault
- NMI (3) comes in.
- nesting detected, popf/ret
- iret faults
- executes only one extra NMI handler

We miss NMI (3) here. I think this is an important change from a semantic where,
AFAIK, the hardware should be allowed to assume that the CPU will execute as
many nmi handlers are there are NMIs acknowledged.

Thanks,

Mathieu

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.