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

[Ingo Molnar]

* H. Peter Anvin <hpa@xxxxxxxxx> wrote:

> 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.

Yeah - it makes no sense for the NMI return indeed, as the target 
CS/SS is hardcoded indeed.

	Ingo
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