Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chainsupport to use NMI-safe methods

[Ingo Molnar]

* Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx> wrote:

> Hrm, would it be possible to save the c2 register upon nmi handler 
> entry and restore it before iret instead ? This would ensure a 
> nmi-interrupted page fault handler would continue what it was 
> doing with a non-corrupted cr2 register after returning from nmi.
> 
> Plus, this involves no modification to the page fault handler fast 
> path.

I guess this kind of nesting would work too - assuming the cr2 can 
be written to robustly.

And i suspect CPU makers pull off a few tricks to stage the cr2 info 
away from the page fault entry execution asynchronously, so i'd not 
be surprised if writing to it uncovered unknown-so-far side-effects 
in CPU implementations.

If possible i wouldnt want to rely on such a narrowly possible hack 
really - any small change in CPU specs could cause problems years 
down the line.

The GUP based method is pretty generic though - and can be used on 
other architectures as well. It's not as fast as direct access 
though.

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/