From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755322AbaIPUTU (ORCPT ); Tue, 16 Sep 2014 16:19:20 -0400 Received: from mail-we0-f171.google.com ([74.125.82.171]:34507 "EHLO mail-we0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753936AbaIPUTQ (ORCPT ); Tue, 16 Sep 2014 16:19:16 -0400 Message-ID: <54189B3D.3040301@gmail.com> Date: Tue, 16 Sep 2014 23:19:09 +0300 From: Nadav Amit User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:31.0) Gecko/20100101 Thunderbird/31.1.0 MIME-Version: 1.0 To: Ingo Molnar , Nadav Amit CC: pbonzini@redhat.com, hpa@zytor.com, mingo@redhat.com, tglx@linutronix.de, x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds , Andrew Morton , Peter Zijlstra , Borislav Petkov Subject: Re: [PATCH 0/3] x86: structs for cpuid info in x86 References: <1410870160-28845-1-git-send-email-namit@cs.technion.ac.il> <20140916132226.GA13726@gmail.com> In-Reply-To: <20140916132226.GA13726@gmail.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 9/16/14 4:22 PM, Ingo Molnar wrote: > > * Nadav Amit wrote: > >> The code that deals with x86 cpuid fields is hard to follow since it performs >> many bit operations and does not refer to cpuid field explicitly. To >> eliminate the need of openning a spec whenever dealing with cpuid fields, this >> patch-set introduces structs that reflect the various cpuid functions. >> >> Thanks for reviewing the patch-set. >> >> Nadav Amit (3): >> x86: Adding structs to reflect cpuid fields >> x86: Use new cpuid structs in cpuid functions >> KVM: x86: Using cpuid structs in KVM >> >> arch/x86/include/asm/cpuid_def.h | 163 +++++++++++++++++++++++++++++++++++++++ >> arch/x86/kernel/cpu/common.c | 56 ++++++++------ >> arch/x86/kvm/cpuid.c | 36 +++++---- >> 3 files changed, 219 insertions(+), 36 deletions(-) >> create mode 100644 arch/x86/include/asm/cpuid_def.h > > I personally like bitfields in theory (they provide type clarity > and abstract robustness, compared to open-coded bitmask numeric > literals that are often used in cpuid using code, obfuscating > cpuid usage), with the big caveat that for many years I didn't > like bitfields in practice: older versions of GCC did a really > poor job of optimizing them. > > So such a series would only be acceptable if it's demonstrated > that both 'latest' and 'reasonably old' GCC versions do a good > job in that department, compared to the old open-coded bitmask > ops ... > > Comparing the 'size vmlinux' output of before/after kernels would > probably be a good start in seeing the impact of such a change. > > If those results are positive then this technique could be > propagated to all cpuid using code in arch/x86/, of which > there's plenty. Thanks for the quick response. I was not aware GCC behaves this way. I made some small experiments with GCC-4.8 and GCC-4.4 and in brief my conclusions are: 1. The assembled code of bitmask and bitfields is indeed different. 2. GCC-4.8 and GCC-4.4 behave pretty much the same, yet GCC-4.8 appears to make better instructions reordering. 3. Loading/storing a single bitfield seems to be pretty much optimized (marginal advantage from code size point-of-view for bitmask, same number of instructions). 4. Loading/storing multiple bitfields seems to be somewhat under-optimized - multiple accesses to the original value result in ~30% more instructions and code-size. So you are correct - bitfields are less optimized. Nonetheless, since cpuid data is mostly used during startup, and otherwise a single bitfield is usually accessed in each function - I wonder whether it worth keeping the optimized but "obfuscate" code. Obviously, I can guess your answer to this question... Nadav