linux-kernel - Re: [PATCH 0/3] x86: structs for cpuid info in x86

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Date:	Tue, 16 Sep 2014 15:22:27 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Nadav Amit <namit@...technion.ac.il>
Cc:	pbonzini@...hat.com, hpa@...or.com, mingo@...hat.com,
	tglx@...utronix.de, x86@...nel.org, kvm@...r.kernel.org,
	nadav.amit@...il.com, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 0/3] x86: structs for cpuid info in x86

* Nadav Amit <namit@...technion.ac.il> 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,

	Ingo
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