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>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...hat.com>, Arnd Bergmann <arnd@...db.de>, Matthias Kaehlcke <mka@...omium.org>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, Juergen Gross <jgross@...e.com>, Paolo Bonzini <pbonzini@...hat.com>, Radim Krčmář <rkrcmar@...hat.com>, Joerg Roedel <joro@...tes.org>, Tom Lendacky <thomas.lendacky@....com>, Andy Lutomirski <luto@...nel.org>, Borislav Petkov <bp@...e.de>, Brian Gerst <brgerst@...il.com>, "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>, "Rafael J . Wysocki" <rjw@...ysocki.net>, Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>, Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...ux.com>, Paul Gortmaker <paul.gortmaker@...driver.com>, Chris Metcalf <cmetcalf@...lanox.com>, Andrew Morton <akpm@...ux-foundation.org>, "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>, Nicolas Pitre <nicolas.pitre@...aro.org>, Christopher Li <sparse@...isli.org>, "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>, Lukas Wunner <lukas@...ner.de>, Mika Westerberg <mika.westerberg@...ux.intel.com>, Dou Liyang <douly.fnst@...fujitsu.com>, Daniel Borkmann <daniel@...earbox.net>, Alexei Starovoitov <ast@...nel.org>, Masahiro Yamada <yamada.masahiro@...ionext.com>, Markus Trippelsdorf <markus@...ppelsdorf.de>, Steven Rostedt <rostedt@...dmis.org>, Kees Cook <keescook@...omium.org>, Rik van Riel <riel@...hat.com>, David Howells <dhowells@...hat.com>, Waiman Long <longman@...hat.com>, Kyle Huey <me@...ehuey.com>, Peter Foley <pefoley2@...oley.com>, Tim Chen <tim.c.chen@...ux.intel.com>, Catalin Marinas <catalin.marinas@....com>, Ard Biesheuvel <ard.biesheuvel@...aro.org>, Michal Hocko <mhocko@...e.com>, Matthew Wilcox <mawilcox@...rosoft.com>, "H . J . Lu" <hjl.tools@...il.com>, Paul Bolle <pebolle@...cali.nl>, Rob Landley <rob@...dley.net>, Baoquan He <bhe@...hat.com>, Daniel Micay <danielmicay@...il.com>, the arch/x86 maintainers <x86@...nel.org>, Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, xen-devel <xen-devel@...ts.xenproject.org>, kvm list <kvm@...r.kernel.org>, Linux PM list <linux-pm@...r.kernel.org>, linux-arch <linux-arch@...r.kernel.org>, Sparse Mailing-list <linux-sparse@...r.kernel.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>, Linus Torvalds <torvalds@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Borislav Petkov <bp@...en8.de> 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
Powered by blists - more mailing lists
Confused about mailing lists and their use? Read about mailing lists on Wikipedia and check out these guidelines on proper formatting of your messages.