linux-kernel - Re: [RESEND PATCH 1/3] x86: Adding structs to reflect cpuid fields

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Date:	Thu, 18 Sep 2014 02:29:54 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Nadav Amit <namit@...technion.ac.il>,
	Ingo Molnar <mingo@...nel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	kvm <kvm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [RESEND PATCH 1/3] x86: Adding structs to reflect cpuid fields

2014-09-17 17:22+0200, Borislav Petkov:
> On Wed, Sep 17, 2014 at 05:04:33PM +0200, Radim Krčmář wrote:
> > which would result in a similar if-else hack
> > 
> >   if (family > X)
> >   	ebx.split.max_monitor_line_size_after_family_X = 0
> >   else
> >   	ebx.split.max_monitor_line_size = 0
> > 
> > other options are
> >   ebx.split.after_family_X.max_monitor_line_size
> > or even
> >   ebx.split.max_monitor_line_size.after_family_X
> 
> And how is that better than simply doing
> 
> 	cpuid = cpuid_ebx(5);
> 
> 	if (family > X)
> 		max_monitor_line_size = cpuid & MASK_FAM_X;
> 	else
> 		max_monitor_line_size = cpuid & MASK_BEFORE_FAM_X;
> 
> ?
> 
> With proper variable naming all is perfectly clear, readable
> and simple. You don't need to open even the CPUID manual - the
> variable tells you you're getting the max monitor line size -
> "ebx.split.max_monitor_line_size_after_family_X" needs me to parse it
> with my eyes first.

I think you proposed to use magic constant in place of of MASK_FAM_X, so
the code above is

  	if (family > X)
  		max_monitor_line_size = cpuid & 0x1ffff;
  	else
  		max_monitor_line_size = cpuid & 0xffff;

We can nicely oneline it, but that's about all the benefits I can see.
It is prone to typos, hard to search for and limiting our operations to
a simple assignment to a properly named variable.

(I prefer descriptive, horribly long, names to raw constant everywhere,
 MASK_MAX_MONITOR_LINE_SIZE_FAM_X.)


Second problem:  Most elements don't begin at offset 0, so the usual
retrieval would add a shift, (repurposing max_monitor_line_size)

 max_monitor_line_size = (cpuid & MASK_FAM_X) >> OFFSET_FAM_X;

and it's not better when we write it back after doing stuff.

 cpuid = (cpuid & ~MASK_FAM_X) | (max_monitor_line_size << OFFSET_FAM_X
                                  & MASK_FAM_X);

All would be fine if we abstracted this with more macros ... wait,
bitfield already does that!

 max_monitor_line_size = cpuid.split.max_monitor_line_size_fam_x;

 cpuid.split.max_monitor_line_size_fam_x = max_monitor_line_size;


---
OT: I'd remove '.split', but we probably wouldn't agree about '.full'.
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