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Date: Thu, 21 Sep 2017 17:59:19 +0200
From: Ingo Molnar <mingo@...nel.org>
To: Thomas Garnier <thgarnie@...gle.com>
Cc: Herbert Xu <herbert@...dor.apana.org.au>,
	"David S . Miller" <davem@...emloft.net>,
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Subject: Re: x86: PIE support and option to extend KASLR randomization


( Sorry about the delay in answering this. I could blame the delay on the merge 
  window, but in reality I've been procrastinating this is due to the permanent,
  non-trivial impact PIE has on generated C code. )

* Thomas Garnier <thgarnie@...gle.com> wrote:

> 1) PIE sometime needs two instructions to represent a single
> instruction on mcmodel=kernel.

What again is the typical frequency of this occurring in an x86-64 defconfig 
kernel, with the very latest GCC?

Also, to make sure: which unwinder did you use for your measurements, 
frame-pointers or ORC? Please use ORC only for future numbers, as
frame-pointers is obsolete from a performance measurement POV.

> 2) GCC does not optimize switches in PIE in order to reduce relocations:

Hopefully this can either be fixed in GCC or at least influenced via a compiler 
switch in the future.

> The switches are the biggest increase on small functions but I don't
> think they represent a large portion of the difference (number 1 is).

Ok.

> A side note, while testing gcc 7.2.0 on hackbench I have seen the PIE
> kernel being faster by 1% across multiple runs (comparing 50 runs done
> across 5 reboots twice). I don't think PIE is faster than a
> mcmodel=kernel but recent versions of gcc makes them fairly similar.

So I think we are down to an overhead range where the inherent noise (both random 
and systematic one) in 'hackbench' overwhelms the signal we are trying to measure.

So I think it's the kernel .text size change that is the best noise-free proxy for 
the overhead impact of PIE.

It doesn't hurt to double check actual real performance as well, just don't expect 
there to be much of a signal for anything but fully cached microbenchmark 
workloads.

Thanks,

	Ingo

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