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

Maciej W. Rozycki
2010-07-14T21:45:11Z

On Wed, 14 Jul 2010, Linus Torvalds wrote:

> No. As mentioned, there is no such counter in real hardware either.

There is a 1-bit counter or actually a latch.

> Look at what happens for the not-nested case:
>
> - NMI1 triggers. The CPU takes a fault, and runs the NMI handler with
> NMI's disabled

Correct.

> - NMI2 triggers. Nothing happens, the NMI's are disabled.

The NMI latch records the second NMI. Note this is edge-sensitive like
the NMI line itself.

> - NMI3 triggers. Again, nothing happens, the NMI's are still disabled

Correct.

> - the NMI handler returns.
>
> - What happens now?

NMI2 latched above causes the NMI handler to be invoked as the next
instruction after IRET. The latch is cleared as the interrupt is taken.

> How many NMI interrupts do you get? ONE. Exactly like my "emulate it
> in software" approach. The hardware doesn't have any counters for
> pending NMI's either. Why should the software emulation have them?

Two. :)

Maciej