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

[Mathieu Desnoyers]

* H. Peter Anvin (hpa@xxxxxxxxx) wrote:
> Linus Torvalds wrote:
> > 
> > On Mon, 15 Jun 2009, Ingo Molnar wrote:
> >> A simple cr2 corruption would explain all those cc1 SIGSEGVs and 
> >> other user-space crashes i saw, with sufficiently intense sampling - 
> >> easily.
> > 
> > Note that we could work around the %cr2 issue, since any corruption is 
> > always nicely "nested" (ie there are never any SMP issues with async 
> > writes to the register).
> > 
> > So what we _could_ do is to have a magic value for %cr2, along with a "NMI 
> > sequence count", and if we see that value, we just return (without doing 
> > anything) from the page fault handler.
> > 
> 
> Wouldn't it be simpler to just require the NMI handler to save and
> restore %cr2 around any potentially faulting references?
> 
> 	-hpa

If we require that around the whole NMI handler execution, then we get
all vmalloc + module text code references handled for free. This would
be a nice-to-have.a And given nmi-handler is not such a frequent code
path, we should not care that much about the performance hit of
saving/restoring the cr2 register at each nmi entry/exit.

Mathieu

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Mathieu Desnoyers
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