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

Linus Torvalds
2010-07-14T21:53:05Z

On Wed, Jul 14, 2010 at 2:45 PM, Maciej W. Rozycki <macro@linux-mips.org> wrote:
> 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.

Well, that's what our single-word flag is too.

>> 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. :)

You just count differently. I don't count the first one (the "real"
NMI). That obviously happens. So I only count how many interrupts we
need to fake. That's my "one". That's the one that happens as a result
of the fault that we take on the iret in the emulated model.

So there is no need to count anything. We take a fault on the iret if
we got a nested NMI (regardless of how _many_ such nested NMI's we
took). That's the "latch", exactly like in the hardware. No counter.

(Yeah, yeah, you can call it a "one-bit counter", but I don't think
that's a counter. It's just a bit of information).

Linus