Messages in this thread | | | Date | Mon, 15 Jun 2009 23:13:19 +0200 | From | Ingo Molnar <> | Subject | Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods |
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* H. Peter Anvin <hpa@zytor.com> 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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