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

["H. Peter Anvin" <hpa@xxxxxxxxx>]

Ingo Molnar wrote:
> 
> hm, does this really work? Using sysret there would be quite 
> tempting. Does anyone know the rough cycle count difference between 
> IRET and SYSRET on contemporary hardware?
> 
> Also, is SYSRET NMI-invariant? If yes then this would be a quite 
> clean all-around solution: on modern hw we'd standardize on doing 
> SYSRET from pretty much all the contexts. We'd get a nice speedup 
> and also the NMI nested pagefaults fix.
> 
> Oh, compat mode. Doesnt SYSRET on Intel CPUs have the problem of not 
> being able to switch back to 32-bit user-space?
> 

Not sure.  SYSRET/SYSEXIT are *not* general return to userspace
solutions in either case; any kind of complex modes and they can't.

And they are, of course, only applicable for returning to userspace.  As
such, I don't understand the "NMI invariant" comment.

	-hpa

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