Re: 64bit x86: NMI nesting still buggy?

[Steven Rostedt]

On Tue, 29 Apr 2014 15:05:55 +0200 (CEST)
Jiri Kosina <jkosina@xxxxxxx> wrote:


> According to 38.4 of [1], when SMM mode is entered while the CPU is 
> handling NMI, the end result might be that upon exit from SMM, NMIs will 
> be re-enabled and latched NMI delivered as nested [2].

Note, if this were true, then the x86_64 hardware would be extremely
buggy. That's because NMIs are not made to be nested. If SMM's come in
during an NMI and re-enables the NMI, then *all* software would break.
That would basically make NMIs useless.

The only time I've ever witness problems (and I stress NMIs all the
time), is when the NMI itself does a fault. Which my patch set handles
properly. I've also stressed this on boxes that do have SMIs and SMMs.

Now, you can have a bad BIOS that does re-enable NMIs from SMMs or
SMIs, but then you need to take that up with your vendor.

-- Steve


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