On Mon, 15 Jun 2009, Ingo Molnar wrote: > > [ If on the other hand it's a speedup of a few cycles then we have > the problem of me suddenly liking this patch a whole lot more ;-) ] I missed the patch. It's quite possible that replacing "iret" with a regular "ret" (for the kernel->kernel transition) is a real speedup. That said, there's a few things to think about: - CPU return stack caches/predictors. I suspect that "iret" and exceptions don't generally touch them (but who knows - maybe they do), while a regular "ret" definitely does. I dunno about "retf". This can cause very subtle performance slowdowns, where the slowdown happens somewhere else. And it could be _very_ uarch-dependent (ie only happen on some architectures, while having no performance downside on others) - kernel->kernel exceptions _should_ be rare, with the exception of actual real external interrupts. So the path to optimize should always be the user-space exception path. That one will need 'iret', but I'd also not want to see more testing in that hot-path. I suspect we already always test for user-mode anyway (due to signal handling etc work), but if it adds new tests to that path, any kernel->kernel speedup is likely totally pointless. That said, it would be nice to avoid 'iret' if only because of its subtle interactions with the while NMI flag. Linus -- To unsubscribe from this list: send the line "unsubscribe linux-tip-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
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